I  I  1 1 


Mai\yland 
Weather  Service 


MARYLAND  WEATHER  SERVICE 


VOLUME  TWO 


MARYLAND 


WEATHER  SERVICE 


VOLUME  TWO 


BALTIMORE 

THE  JOHNS  HOPKINS  PRESS 

1907 


Z^i  Boxi  (§aitimovt  (prcee 

BALTIMORE,  MD.,  V.  S.  A. 


BOARD  OF  CONTROL 

W.A[.  BULLOCK   CLAKK, Director. 

REPRESEM'INC;    THE    JOII>"S    HOPKIXS    UMVKKSITY. 

W.   T.   L.  TALIAFERRO,        .         .         Secretary  axd  Treasurer. 

REPRESENTING   THE   MARYLAND   AGRICULTURAL   COLLEGE. 

OLREK    LAXARt)   FA.SSIG,,       ....         Meteohologist. 

REPRESENTING    THE    U.    S.    WEATHER    BUREAU. 


TJie  Maryland  Weather  Service  is  conducted  under  the  joint  auspices 
of  the  institutions  above  mentioned,  the  Central  OflBce  being  located  at 
the  Johns  Hopkins  University.  The  meteorological  work  is  under  the 
immediate  supervision  of  the  Meteorologist  who  is  detailed  by  the  Chief 
of  the  U.  S.  Weather  Bureau.  Other  lines  of  investigation  are  carried 
on  in  co-operation  with  various  State  and  National  organizations. 


LETTER  OF  TRANSMITTAL 

To  His  Excellenc}',  Edwin  Warfield, 
Governor  of  Maryland, 
Sir: — I  have  the  honor  to  present  herewith  the  second  volume  of  the 
new  series  of  reports  of  the  Maryland  Weather  Service.  The  first  volume 
contained  a  general  account  of  the  physiography  and  meteorology  of  the 
State  while  the  present  volume  is  chiefly  devoted  to  a  special  study  of  the 
climatic  features  of  Baltimore  and  vicinity.    I  am, 

Very  respectfully, 

Wm.  Bullock  Clark, 

Director. 

Johns  Hopkins  Univebsitt, 
December  1,  1907. 


CONTENTS 


^  PAGE 

PREFACE     17 

INTRODUCTION,  OPERATIONS  OF  THE  SERVICE.     By  Wm.  Bullock 

Clark     21 

Physiography  and  Climate  of  the  State 21 

Climate  and  Weather  of  Baltimore 22 

Climate  of  the  Counties 22 

Distribution  of  Plant  Life  in  the  State 23 

SL'B\^y  of  the  Swa:mp  Lands  of  the  State 23 

Other  Lines  of  Work 25 

THE    CLIMATE    AND    WEATHER    OF    BALTIMORE.     By    Oliver    L. 

Fassig     27 

THE  CLIMATE  OF  BALTIMORE 29 

Introduction    29 

The  Geographic  Horizon  of  Baltimore 30 

Atmospheric  Pressure    31 

The  Diurnal  Variations  of  the  Barometer 34 

The  Normal  Diurnal  Variation  at  Baltimore 34 

Phases  of  Diurnal  Oscillation 35 

Diurnal  Variations  of  Pressure  on  Clear  and  Cloudy  Days 40 

The  Diurnal  Barometric  Wave 41 

Corrections  for  Reduction  to  true  Mean  Pressure 43 

The  Annual  March  of  Atmospheric  Pressure 44 

Average  Monthly  and  Annual  Pressure 47 

Annual  and  Secular  Variations  of  Pressure 50 

The  Average  Variability  of  Pressure 53 

Extremes  of  Pressure Sfi 

Temperature  of  the  Atmospheke 56 

Introduction     o6 

Average  Temperatures    57 

The  Normal   Hourly   Temperature 59 

Phases  of  the  Diurnal  Variation 63 

Diurnal  Variation  as  Affected  by  Clouds  and  Rain 66 

Effect  of  a  Snow  Covering 69 

The  Effect  of  Wind  Velocity  on  Temperature 70 


CONTENTS 

PAGE 

Range  of  Temperature  on  Calm  and  Windy  Days 72 

Reduction  to  the  True  Mean  Temperature 72 

The  Hourly  Rate  of  Change 73 

Mean  Daily  Temperature 76 

Average  Inter-diurnal  Changes  of  Temperature 79 

Average  Daily  Range 83 

Diurnal  Variability  of  Temperature 83 

The  Probable  Error  of  the  Mean  Daily  Temperatures 90 

Mean  Monthly,  Seasonal,  and  Annual  Temperatures 91 

The  Normal  Temperature 95 

The  Variability  of  the  Monthly  and  Annual  Mean 96 

Warm  Months  and  Seasons 97 

Frequency   of   Stated    Departures    from    the    Monthly    Seasonal    and 

Annual  Mean  Temperatures 99 

The  Probable  Error  of  the  Monthly  and  Annual  Means 101 

Succession  of  the  Seasons 103 

Daily  Extremes   of   Temperature 104 

The  Greatest  Daily  Range  of  Temperature 109 

Monthly  and  Annual  Extremes 112 

The  Greatest  Monthly  Range 114 

Frequency  of  Days  with  Frost 115 

The  Frequency  of  Cold  Waves 126 

Killing  Frdsts    129 

The  First  and  Last  Occurrence  of  a  Minimum  of  32° 131 

Light   Frosts    135 

The  Period  of  Effective  Temperatures  for  Plant  Growth 136 

The  Frequency  of  Warm  Days  in  Summer 137 

Time  of  Occurrence  of  Annual  Minimum  and  Maximum  Temperatures  145 

Temperature  of  the  Water  in  the  Harbor 147 

Humidity    148 

Introduction     148 

Hourly  Variation  in  Humidity 152 

Phases  of  the  Diurnal  March  of  Relative  Humidity 154 

Mean  Monthly  and  Annual  Relative  Humidity 156 

Absolute  Humidity    158 

Mean  Vapor  Pressure 159 

Pkecipitation     159 

Introduction     159 

The  Causes  of  Precipitation 161 

The  Geographical  Distribution  of  Rainfall 161 

The  Influence  of  Wind  Direction 162 

The  Influence  of  Topography 162 

The  Influence  of  Atmospheric  Pressure 163 

The  Seasonal  Distribution  of  Rainfall 164 

Hourly  Amount  of  Rainfall 165 


MARYLAXD   WEATHER   SEKYICE  7 

PAGE 

Hourly    Rainfall    Frequency 167 

Duration  of  Precipitation 170 

Frequency  of  Precipitation  of  Stated  Amounts 174 

Average  Daily  Rainfall 178 

Daily  Rainfall  Frequency 182 

The  Probability   of  Rain 183 

The  Monthly  Precipitation    185 

The  Seasonal  and  Annual  Precipitation 190 

Monthly  and  Annual  Departures 195 

Excessive  Rains    197 

Greatest  Rainfall   in   24  Hours 199 

Excessive  Rates  of  Precipitation 205 

Dry   Spells 214 

Wet  Spells    219 

The  Distribution  of  Precipitation  in  Normal,  Dry,  and  Wet  Years.  .  .  223 

Snowfall      227 

Dates  of  First  and  Last  Snow 231 

The  Frequency  of  Days  with  Snowfall 232 

Heavy    Snowfalls    235 

Duration  of  Snowfall 236 

Fogs     237 

Sunshine  and  Cloudiness 239 

Sunshine     239 

Average  Daily   Sunshine 243 

Sunshine  Phases     244 

Cloudiness    245 

Clear,  Partly  Cloudy,  and  Cloudy  Days 246 

Frequency  of  Clear  Days 248 

Frequency  of  Partly  Cloudy  Days 250 

Cloudy   Days    251 

The   Winds    251 

Introduction     251 

Average  Hourly  Wind  Movement 252 

Average  Daily  and  Total  Monthly  Wind  Movement 255 

Maximum  Wind  Velocities 258 

Frequency  and  Duration  of  Stated  Wind  Velocities 261 

Average  Duration  of  Storm  Winds 262 

Gales    263 

Prevailing  Hourly  Wind  Directions 265 

Prevailing  Monthly  and  Annual   Directions 268 

Monthly  Frequency  of  Stated  Directions 273 

The  Direction  of  Upper  and  Lower  Clouds 274 

Electrical  Phenomena   276 

Thunderstorms      276 

Thunderstorm    Probability    280 


8  CONTENTS 

PAGE 

Consecutive   Days   with   Thunderstorms 280 

Direction  of  Thunderstorms 281 

Pressure  Changes  during  Thunderstorms 282 

Hail    284 

Auroras      288 

Sunspots  and  Weather 288 

General  Character  of  the  Seasons 295 

Observations   and   Instrumental   Equipment 296 

Historical  Notes    296 

Observers  and   Observations 301 

Instrumental  Equipment    301 

Hours    of    Observation 302 

Changes  in  the  Location  of  the  Station  and  Officials  in  Charge 304 

Summary  of  Average  and  Extreme  Values 306 

THE  WEATHER  OF  BALTIMORE 311 

Introduction     311 

The    Synoptic    Weather    Chart 312 

Cyclones    and    Anti-cyclones 313 

Areas  of  Unsettled  Weather    (Cyclones) 316 

Pressure   and    Winds 318 

Temperature   and   Wind    Direction 319 

Distribution  of  Clouds  and  Precipitation 320 

Areas  of  Fair  Weather   (Anti-cyclones) 321 

Isobars    and    Winds 321 

The  Winds  and  Distribution  of  Temperature 323 

Distribution   of  Clouds 324 

The  Eastward  Drift  of  Cyclones  and  Anti-cyclones 324 

Weather  Charts  of  the  Northern  Hemisphere 327 

Weather  of  the  Principal  Climatic  Zones 328 

The  Tropical  Zone 328 

The  Temperate  Zones 329 

The    Polar    Zones ; 330 

The    Seasons     331 

AVinter  Weather 333 

Winter  Cyclones    334 

The    Lake    Storm 335 

The  Storm  of  December  24-26,  1902 335 

The  Storm  of  January  7-8,  1903 341 

The  Storm  of  February  27-March  1,  1903 345 

The    Southwest    Storm 350 

The  Storm  of  February  3-5,  1903 350 

The  Storm  of  December  26-28,   1904 354 

The  Storm  of  December  12-13,  1903 359 


MARTLAXD    W  EATHEll    SEilVICE  9 

PAGE 

The    Gulf    Storm 363 

The  Storm  of  February  1-3,  1902 364 

The  Storm  of  January  5-7,  1905 368 

The  Storm  of  February  20-22,   1902 373 

The    Blizzard    378 

The  Blizzard  of  March  11-13,  1888 378 

The  Blizzard  of  February  12-14,  1899 382 

Areas  of  Fair  Weather    (Anti-cyclones) 389 

Cold    Waves    391 

The  Cold  Wave  of  December  13-15,  1901 ^ 392 

The  Cold  Wave  of  February  10-13,  1899 395 

The  Origin  of  Cold  Waves 396 

The  Cold   Winter  of   1903-04 397 

The  Warm   \\'inter  of   1889-90 399 

The  Distribution  of  Atmospheric  Pressure  during  the  Cold  Winter 

of  1903-04  and  the  Warm  Winter  of  1889-90 400 

The  Variability   of  Winter   Weather 401 

The  Weather  of  Christmas  Day 404 

The  AVeather  of  Washington's  Birthday 409 

Si'Ki.Nc;   Wkathek    410 

March    Winds    and    Storms 412 

Ice    Storms    413 

The  Squall  of  March  1 ,  1907 413 

Equinoctial    Storms    415 

Hail   Storms    417 

The  Storm  of  May  19,  1904 418 

The  Storm  of  April  27,  1890 418 

Spring    Frosts    421 

Ice  without  Frost 423 

Periods  of  Unsettled  Weather 424 

The  Rainy  Period  of  April  19-25,  1901 425 

The  Rainy  Period  of  May  16-26,  1894 426 

The  Variability  of  Weather  in  Spring 428 

The   Weather  of   March   4 429 

The  Weather  of  May  1 432 

The  Weather  of  Easter   Sundays 432 

Sim  .mi;k    Wkathki!    436 

Summer   Storms    437 

The   Thunderstorm   of  July   20,   1902 438 

Ttic  'j'hunderslorm  of  July  3,  1902 444 

The   Thunderstorm  of  July  12,   1904 446 

The   Tornado   of   July   12,    1903 447 

*     Waterspouts     452 

Summer  Hot  Spells 453 

The  Summer  of  1900 454 

General   Weather  Conditions 459 


10  CONTENTS 

PAGE 

The  Summer  of  1901 462 

The  Hot  Periods  of  August,  1900,  and  July,  1901,  Compared 463 

Days  with  a  Maximum  Temperature  of  90°  or  above 466 

The  Cold  Summer  of  1816 467 

Distribution  of  Pressure  during  the  Cool  June  of  1903 469 

Distribution  of  Pressure  during  the  Normal  June  of  1902 470 

The  Variability  of  Summer  Temperatures 470 

The   Weather    of   July    4 472 

West   Indtan   Hurricanes 475 

Frequency    of    Hurricanes 476 

The  Hurricane  of  October  13,  1893 476 

Autumn  Weather    480 

Indian  Summer  482 

The  Variability  of  Autumn  Temperatures 486 

The  Weather  of  September  12 486 

The  Weather  of  October  1 489 

The   Weather   of   Thanksgiving   Day 489 

The  Heavy  Rains  of  September  24-26,  1902 492 

FOEETEtLING   THE   WEATHEB    (HISTORICAL) 493 

Introduction    493 

Natural    Signs    494 

Astro-Meteorology    496 

Symbolic   Days    498 

Early  Books  on  Weather  Proverbs 498 

Forecasts  Based  on  Average  and  Extreme  Values 499 

Temperature    Variability    500 

Rainfall    Probability 500 

Special  Days  501 

Recurring   Periods    502 

The  Method  of  the  Synoptic  Weather  Chart 504 

The  Indian  Seasonal  Forecasts 507 

Index    511 


ILLUSTRATIONS 


PLATE  FACIXG   PAGE 

I.  The  Diurnal  Barometric  Wave 42 

II.  Typical  Barograms    44 

III.  Daily  March  of  Temperature  and  Pressure 80 

IV.  Daily  March  of  Temperature 82 

V.  Typical   Thermograms    86 

VI.     Departures  of  Mean  Monthly  Temperature  from  Normal  for  87 

Years     87-88 

VII.     Departures  of  Mean  Monthly,   Seasonal,  and   Annual   Tempera- 
ture from  Normal  for  87  Years 92 

VIII.     Selected   Relative  Humidity   Curves 158 

IX.     Precipitation  Probability     184 

X.     Monthly,   Seasonal,   and   Annual   Departures    from   the    Normal 

Precipitation    (1817-1904)     194 

XI,     Average  Hourly  Wind  Direction 266 

XII.     Sunspots,  Solar  Prominences,  and  Weather  Conditions 294 

XIII.  General  Character  of  the  Seasons. — Winter 296 

XIV.  General  Character  of  the  Seasons. — Spring 296 

XV.     General  Character  of  the  Seasons. — Summer 296 

XVI.     General  Character  of  the  Seasons. — Autumn 296 

XVII.     General  Character  of  the  Year 296 

XVIII.     Office  of  the  U.  S.  Weather  Bureau  and  Maryland  State  Weather 

Service     300 

XIX.     Storm  Warning  Display  Station 304 

XX.     Hourly  Observations  at  Baltimore  during  the  Blizzard  and  Cold 

Wave  of  Feb.  9-14,  1899 388 

XXI.     Frost  Figures    311 

XXII.     Effects  of  Ice  Storm  of  March,  1906 413 

XXIII.  Depanures  in  Temperature  during  the  Hot  Spell  of  1900 462 

XXIV.  Distribution  of  Pressure,  Winds,  and  Temperature  during  Normal, 

Cold,  and  Warm  Seasons  in  the  United  States 470 

FIGURE  ■  PAGE 

1.  Hourly  Variations  of  the  Barometer 33 

2.  Isopleths  of  Hourly  Pressure 36 

3.  Principal  Phases  of  Diurnal  Oscillation  of  Pressure 38 

4.  Diurnal  Variations  of  Pressure  on  Clear  and  on  Cloudy  Days 40 

5.  The  Diurnal  Barometric  Wave 42 


12  ILLCSTRATIOXS 

PAGE 

6.  Mean  Monthly  Atmospheric  Pressure 44 

7.  Variations  in  the  Mean  Monthly  Pressure 46 

8.  Annual  Variations  of  Pressure   Expressed  as   Departures   from  the 

Normal  Value    50 

9.  Monthly  Means  and  Extremes  of  Pressure 54 

10.  Mean    Hourly   Temperature 60 

11.  Isopleths  of  Hourly  Temperature 62 

12.  Principal  Phases  of  Diurnal  Variation  of  Temperature 64 

13.  Effect  of  Cloudiness  and  Rain  on  the  Hourly  Variations  of  Tempera- 

ture          67 

14.  Effect  of  Snow-covering  on  the  Hourly  Variations  of  Temperature..     68 

15.  Effect  of  Wind  Velocity  on  the  Hourly  Variations  of  Temperature..     71 

16.  Hourly  Rate  of  Change  of  Temperature 74 

17.  Curves  Representing  the  Average  Hourly  Pressure  and  the  Hourly 

Rate  of  Change  in  Temperature  for  the  Year 76 

IS.     Inter-diurnal    Temperature   Changes 80 

19.  Total  Seasonal  and  Annual  Frequency  of  Stated  Diurnal  Changes  of 

Temperature     84 

20.  Diurnal   Changes   of   Temperature   of   less   than   6°,    +6°,   +8°,   and 

-1-10°   each  month 85 

21.  Diurnal  Changes  of  Temperature  of  —6°,  -^6°,  +8°,  -M0°,  -|-20°...     88 

22.  Frequency   of    Stated    Departures   from   the   Monthly   Normal   Tem- 

perature        101 

23.  Frequency  of  Stated  Departures  from  the  Normal  Seasonal  and  An- 

nual   Temperatures    102 

24.  Greatest  Daily  Range  of  Temperature 109 

25.  Extreme,  Average,  and  Mean  Maximum  and  Minimum  Temperatures.   Ill 

26.  Absolute  Maximum  and  Minimum  Temperatures 114 

27.  Greatest  Monthly  Range  of  Temperature 115 

28.  Longest  Period  of  Consecutive  Days  with  a  Minimum  Temperature 

of  32°   or  Below 119 

29.  Number  of  Days  with  Mean  Temperature  Below  14°  and  32° 120 

30.  Annual  Frequency  of  Days  with  a  Maximum  Temperature  Below  32°  121 

31.  Annual  Frequency  of  Cold  Days 122 

32.  Monthly  Frequency  of  Cold  Days 122 

33.  Interval   Between  Last  and  First  Occurrence  of  a  Minimum   Tem- 

perature   of    32° 132 

34.  Interval  Between  Last  and  First  Occurrence  of  Minimum  Tempera- 

ture of  40° 134 

■  35.     Annual  Number  of  Days  with  Mean  Temperature  Above  42° 136 

36.  Annual  Number  of  Days  with   Maximum   Temperature   of  90°   and 

Over    137 

37.  Time    of    Occurrence    of    the    Lowest    and    Highest    Temperature    of 

the  Year    144 

38.  Air  and  Water  Temperatures  in  Baltimore  Harbor 144 

39.  Mean  Hourly  Relative  Humidity 151 


:martlaxd  weather  service  13 

PAGE 

40.  Mean  Hourly  Relative  Humidity 153 

41.  Phases  of  the  Diurnal  Variations  in  Relative  Humidity 155 

42.  The  Mean  Monthly  Relative  Humidity 156 

43.  Variations  in  the  Mean  Annual  Relative  Humidity 156 

44.  Average    Hourly    Precipitation 165 

45.  Average  Hourly  Amounts  of  Precipitation  in  January  and  July 167 

46.  Average  Hourly  Frequency  of  Precipitation  in  January  and  July.  . . .  169 

47.  Average  Hourly  Frequency  of  Precipitation 170 

48.  The  Average  Duration  of  Precipitation 172 

49.  Variations  in  the  Annual  Frequency  of  Days  with  Appreciable  Pre- 

cipitation       177 

50.  Monthly   Frequency   of   Precipitation 182 

51.  The  Monthly  Amount  of  Precipitation 185 

52.  Mean   Monthly  Precipitation 188 

53.  Variations  in  the  Annual  Amount  of  Precipitation  for  1817  to  1904.  .  191 

54a.  Departures  from  Mean  Monthly  Precipitation   (1817-1859) 193 

54b.  Departures  from  Mean  Monthly  Precipitation   (1860-1904) 194 

55.  The  Heaviest  Precipitation  in  any  24  Consecutive  Hours 201 

56.  Rainfalls  Equalling  or  Exceeding  2.50  Inches  in  a  Day 202 

57.  Rainfalls  Equalling  or  Exceeding  One  Inch  per  Hour 203 

58a.  Excessive  Rates  of  Rainfall 210 

58b.  Excessive  Rates   of  Rainfall 211 

59.  Dry    Periods    215 

60.  Dry   Periods    218 

61.  Wet   Periods    220 

62.  Total  Monthly  Precipitation  During  a  Dry  Year,  a  Normal  Year,  and 

a  Wet   Year 224 

63.  Daily  Precipitation  During  a  Dry  Year,  a  Normal  Year,  and  a  Wet 

Year    225 

64a.  Annual  Frequency  of  Days  with  a  Snowfall  to  the  Amount  of  One- 
tenth  of  an   Inch 228 

64b.  Annual  Depth  of  Snowfall  in  Inches 229 

65.  Monthly  Frequency  and  Amount  of  Snowfall 233 

66.  Mean  Hourly  Sunshine 241 

67.  Mean  Hourly  Sunshine  for  the  Year 242 

68.  Average  Hourly  Cloudiness 245 

69.  Relative  Frequency  of  Clear.  Partly  Cloudy,  and  Cloudy  Days 247 

70.  Hourly  and  Annual  Variations  of  Wind  Velocity 253 

71.  Average  Hourly  Variations  in  Wind  Velocity 254 

72.  The  Frequency  of  Storm  Winds 258 

73.  Average  Hourly  Wind  Direction facing  page  266 

74.  Prevailing  Morning  and  Afternoon  Wind  Directions  in  January.  .  .  .  267 

75.  Relative  Frequency  of  Prevailing  Wind  Directions 269 

76.  Prevailing   Monthly   Directions  of   the  Wind   in   Warm,    in   Normal, 

and  in  Cold  Seasons  and  Years 270 

77.  The  Frequency  and  Distribution  of  Thunderstorms 277 

78.  The  Average  Monthly  P'requency  of  Occurrence  of  Thunderstorms   .,  277 


14  ILLUSTRATIONS 

PAGE 

79.  The  Annual  Frequency  of  Occurrence  of  Thunderstorms  from  1871 

to  1904    278 

80.  The  Direction  of  Movement  of  Thunderstorms 281 

81.  Some  Typical  Barograms  during  Thunderstorms  and  Squalls 283 

82.  The  Frequency  of  Occurrence  and  the  Hourly  and  Seasonal  Distribu- 

tion   of    Hailstorms 286 

83.  Barograms   during   Hailstorms 286 

84.  Sunspots,  Solar  Prominences,  and  Weather  Conditions 294 

85.  Typical  Cyclone  of  Dec.  27,  1904  (Pressure  and  Winds) 317 

86.  Typical  Cyclone  of  Dec.  27,  1904  (Complete  Chart) 317 

87.  Typical  Anticyclone  of  April  4,  1904  (Pressure  and  Winds) 322 

88.  T3T)ical  Anticyclone  of  April  4,  1904  (Complete  Chart) 322 

89.  Typical  Cyclone  and  Anticyclone  of  March  3,  1904 325 

90.  Pressure  Distribution  over  the  Northern  Hemisphere,  Dec.  4,  1886 . .  327 

91.  The  Lake  Storm  of  Dec.  24,  1902 336 

92.  The  Lake  Storm  of  Dec.  25,  1902 336 

93.  The  Lake  Storm  of  Dec.  26,  1902 337 

94.  The  Lake  Storm  of  Dec.  24-26,  1902   (Diagr.) 339 

95.  The  Lake  Storm  of  Jan.  7,  1903 342 

96.  The  Lake  Storm  of  Jan.  8,  1903 342 

97.  The  Lake  Storm  of  Jan.  6-8,  1903  (Diagr.) 343 

98.  The  Lake  Storm  of  Feb.  27,  1903 346 

99.  The  Lake  Storm  of  Feb.  28,  1903 346 

100.  The  Lake  Storm  of  March  1,  1903 347 

101.  The  Lake  Storm  of  Feb.  27-March  1,  1903  (Diagr.) 348 

102.  The  Southwest  Storm  of  Feb.  3,  1903 351 

103.  The  Southwest  Storm  of  Feb.  4,  1903 352 

104.  The  Southwest  Storm  of  Feb.  5,  1903 352 

105.  The  Southwest  Storm  of  Feb.  3-6,  1903  (Diagr.) 353 

106.  The  Southwest  Storm  of  Dec.  26,  1904 356 

107.  The  Southwest  Storm  of  Dec.  27,  1904 356 

108.  The  Southwest  Storm  of  Dec.  28,  1904 357 

109.  The  Southwest  Storm  of  Dec.  26-28,  1904  (Diagr.) 358 

110.  The  Southwest  Storm  of  Dec.  12,  1903 360 

111.  The  Southwest  Storm  of  Dec.  13,  1903 360 

112.  The  Southwest  Storm  of  Dec.  12-14.  1903  (Diagr.) 361 

113.  Paths  and  Rain  Areas  of  Southwest  Storms  of  Jan.,  1898 362 

114.  The  Gulf  Storm  of  Feb.  1,  1902 365 

115.  The  Gulf  Storm  of  Feb.  2,  1902 365 

116.  The  Gulf  Storm  of  Feb.  3,  1902 366 

117.  The  Gulf  Storm  of  Feb.  1-3,  1902  (Diagr.) 367 

118.  The  Gulf  Storm  of  Jan.  5,  1905 369 

119.  The  Gulf  Storm  of  Jan.  6,  1905 369 

120.  The  Gulf  Storm  of  Jan.  7,  1905 370 

121.  The  Gulf  Storm  of  Jan.  5-7,  1905   (Diagr.) 371 

122.  The  Gulf  Storm  of  Feb.  20,  1902 373 

123.  The  Gulf  Storm  of  Feb.  21,  1902 374 


MARYLAXD    WEATHER    SERVICE  15 

PAGE 

124.  The  Gulf  Storm  of  Feb.  22,  1902 374 

125.  The  Gulf  Storm  of  Feb.  20-22,  1902  (Diagr.) 375 

126.  Paths  of  the  Gulf  Storms  of  February,  1902 376 

127.  Diagram  of  Rainj^  Sundays  of  the  Winter  of  1801-2   (Diagr.) 377 

128.  The  Blizzard  of  March  11,  18SS 379 

129.  The  Blizzard  of  March  12,  1888 379 

130.  The  Blizzard  of  March  13,  1888 380 

131.  The  Blizzard  of  March  11-13,  1SS8   (Diagr.) 381 

132.  The  Blizzard  of  Feb.  9,  1899 384 

133.  The  Blizzard  of  Feb.  10,  1899 384 

134.  The  Blizzard  of  Feb.  11,  1899 385 

135.  The  Blizzard  of  Feb.  12.  1899 385 

136.  The  Blizzard  of  Feb.  13,  1S99 386 

137.  The  Blizzard  of  Feb.  14,  1899 386 

138.  Snow  on  the  Ground  after  the  Blizzard  of  February,  1899 387 

139.  Cold  Wave  of  Dec.  13,  1901 393 

140.  Cold  Wave  of  Dec.  14,  1901 393 

141.  Cold  AVave  of  Dec.  15,  1901 394 

142.  Cold  February  11,  1899 402 

143.  Warm   February  11,  1887 402 

144.  The  Weather  of  Christmas  Day  (December  25)    (Diagr.) 406 

145.  The  W^eather  of  Washington's  Birthday   (February  22)    (Diagr.)..   408 

146.  The  Squall  of  :\Iarch  1,  1907 414 

147.  The  Hail  Storm  of  May  19,  1904 419 

148.  The  Hail  Storm  of  May  19,  1904  ( Diagr. ) 419 

149.  The  Frost  of  May  9,  1906 422 

150.  Ice  without  Frost,  April  17,  1905 424 

151.  The  Weather  of  March  4    (Diagr.) 430 

152.  The  Weather  of  May  1   (Diagr.) 433 

153.  The  Thunderstorm  of  July  20,  1902 440 

154.  The  Thunderstorm  of  July  20,  1902   (Diagr.) 441 

155.  The  Movements  of  the  Thunderstorm  of  July  20,  1902  (Diagr. "i 442 

156.  The  Thunderstorm  of  July  3,  1902 444 

157.  The  Thunderstorm  of  July  3,  1902   (Diagr.) 445 

158.  The  Thunderstorm  of  July  12,  1904 447 

159.  The  Tornado  of  July  12,  1903  (8  a.  m.) 448 

160.  The  Tornado  of  July  12,  1903   (8  p.  m.) 448 

161.  Chart  of  August  6,  1900  (during  Hot  Spell) 460 

162.  Temperature  during  Hot  Spells  of  1900  and  1901   (Diagr.) 464 

163.  The  Cold  July  1,  1885 471 

164.  The  Warm   July  1,   1901 471 

165.  The  Weather  of  July  4    (Diagr.) 474 

166.  The  Hurricane  of  Oct.  13,  1893   (8  a.  m.) 478 

167.  The  Hurricane  of  Oct.  13,  1893  (8  p.  m.) 478 

168.  The  Hurricane  of  Oct.  14,  1893   (8  a.  m.) 479 

169.  The  Weather  of  Oct.  29,  1903   (Indian  Summer) 484 

170.  The  Weather  on  September  12   (Defenders'  Day)    (Diagr.) 488 


PREFACE 

The  present  volume  is  the  second  of  a  series  of  reports  dealing  with 
the  climatic  features  of  Maryland.  The  first  volume  was  general  in 
character  and  presented  all  that  was  then  known  regarding  the  physi- 
ography and  meteorolog}-  of  the  State.  The  present  and  succeeding 
volumes  will  be  devoted  to  more  special  studies  within  the  province  of 
climatological  research. 

The  Introduction  to  the  present  volume,  prepared  by  Wm.  Bullock 
Clark,  is  devoted  chiefly  to  an  account  of  the  operations  of  the  Service 
together  with  the  plans  for  future  work.  An  account  is  given  of  the 
Swamp  Lands  of  the  State  whicli  are  atti-aeting  wide  attention.  The 
writer  refers  to  the  Botanical  Survey  of  the  State,  which  has  been  made 
under  the  auspices  of  the  State  Weather  Service,  the  results  of  which 
will  be  shortly  printed  in  Volume  III  of  the  present  series. 

The  Report  on  the  Climate  and  WeatJier  of  Baltimore  and  Vicinity, 
discussed  by  Oliver  L.  Fassig,  constitutes  the  chief  portion  of  the  vohune 
and  represents  the  result  of  many  years  of  exhaustive  study  of  the  Balti- 
more region.  All  of  the  available  records  both  public  and  private  have 
been  employed  in  this  work  and  the  result  may  be  regarded  as  remarkably 
complete.  It  is  doubtful  if  any  district  luis  received  as  thorough  study 
as  Dr.  Fassig  has  given  to  that  of  Baltimore  and  vicinity.  The  report 
is  divided  into  two  parts.  The  first  deals  with  the  average  and  extreme 
values  of  the  meteorological  elements  recorded  in  the  city  of  Baltimore. 
The  discussion  is  based  upon  careful  ol)Servatious  extending  over  a  period 
of  nearly  a  century.  The  second  part  deals  with  types  of  weather  experi- 
enced in  Baltimore  and  vicinity — hence  with  the  actual  physical  condi- 
tion of  the  atmosphere  at  stated  times,  during  the  prevalence  of  storms, 
cold  and  warm  waves,  etc. 


18  PREFACE 

The  Maryland  Weather  Service  desires  especially  to  extend  its  thanks 
to  Professor  Willis  L.  Moore,  Chief  of  the  U.  S.  Weather  Bureau,  who 
has  generously  aided  the  conduct  of  the  investigations  discussed  in  the 
present  volume.  Dr.  Fassig  has  had  access  to  the  complete  records  of  the 
U.  S.  Weather  Bureau  as  well  as  to  those  of  other  official  organizations. 

Mr.  E.  W.  Berry,  of  the  State  Geological  Survey,  has  materially  aided 
in  editing  the  manuscripts  for  the  volume. 


INTRODUCTION 


OPERATIONS  OF  THE  SERVICE 


BY 

WM.  BULLOCK  CLARK 


INTRODUCTION 
OPERATIONS  OF  THE  SERVICE 

BY 

WM.   BULLOCK  CLARK 


The  Maryland  Weather  Service  has  been  engaged  for  many  years  in  a 
study  of  the  climatic  features  of  Maryland.  These  investigations  have 
resulted  in  the  accumulation  of  a  vast  amount  of  information  relating  to 
the  meteorology,  the  physiography,  the  agricultural  soils,  and  the  distri- 
bution of  plant  life.  Much  aid  has  been  rendered  the  State  Weather 
Service  in  this  work  by  both  the  National  and  State  bureaus. 

Physiography  and  Climate  of  the  State. 

The  results  of  the  physiographic  and  meteorological  studies  of  the 
entire  state  down  to  1899  were  presented  in  Volume  I  of  this  series  of 
reports.  These  investigations  were  based  on  all  the  then  existing  obser- 
vations and  records,  both  official  and  private.  The  physiographic  studies 
had  been  conducted  largely  under  the  auspices  of  the  State  Geological 
Survey,  but  as  the  results  were  so  fundamental  to  an  interpretation  of 
the  climatic  features  of  the  State,  their  publication  in  the  very  first 
volume  of  the  new  series  of  reports  seems  desirable. 

The  meteorological  data  relating  to  Maryland  climate  had  been  ac- 
cumulated for  over  a  century,  but  little  attempt  had  been  made  hitherto 
to  draw  conclusions  from  them  or  to  seek  an  explanation  for  the  many 
variations  that  are  found  in  the  different  sections  of  the  state  and  in  the 


T-i  INTRODUCTION 

same  regions  at  different  seasons  of  the  year.  These  studies  may  be 
regarded  as  preliminary  to  the  more  exhaustive  investigations  which  have 
followed,  as  well  as  to  those  which  still  await  completion. 

Climate  and  Weather  of  Baltimore. 

The  investigations  of  the  climate  and  weather  of  Baltimore  and  vicin- 
ity, discussed  in  the  pages  of  the  present  volume,  may  be  regarded  as 
fully  meeting  the  requirements  of  such  a  detailed  study.  The  author  has 
treated  exhaustively  the  elements  entering  into  the  interpretation  of  the 
conditions  found  to  prevail  in  the  Baltimore  region.  It  is  probably  the 
most  complete  study  that  has  ever  been  given  to  the  climate  and  weather 
of  a  single  city  and  its  environs,  and  will  afford  a  most  important  store- 
house of  information  for  those  who  may  be  seeking  for  an  accurate  knowl- 
edge of  the  exact  conditions  that  prevail  in  Baltimore  and  its  immediate 
surroundings.  The  aid  rendered  by  the  Chief  of  the  U.  S.  Weather 
Bureau  has  alone  made  it  possible  to  secure  the  results  here  recorded. 

Climate  of  the  Counties. 

Special  reports  on  the  climate  of  Allegany,  Garrett,  Cecil,  Calvert,  and 
St.  Mary's  counties  have  been  prepared  by  the  Maryland  Weather  Service 
and  issued  under  the  auspices  of  the  Maryland  Geological  Survey  in  its 
series  of  county  reports.  It  is  the  intention  of  the  State  Weather  Service 
ultimately  to  bring  together  and  publish  these  chapters  when  complete 
for  all  the  counties  in  a  single  volume  of  the  State  Weather  Service 
s'eries.  When  brought  out  this  report  on  the  climate  and  weather  of  the 
Mar3dand  counties  will  present  for  each  political  district  of  the  State  an 
exhaustive  discussion  of  its  special  features  that  will  be  of  great  benefit 
to  the  inhabitants  and  to  those  seeking  information  regarding  the  special 
climatic  conditions  of  any  particular  county.  This  study  will  take  sev- 
eral years  for  its  consummation,  but  with  the  co-operation  so  generously 
furnished  by  the  Chief  of  the  U.  S.  Weather  Bureau  it  will  be  finally 
completed  in  a  form  that  will  be  recognized  as  thoroughly  authoritative. 


MARYLAND   WEATHER    SERVICE  23 

The  Meteorologist  in  charge  of  the  State  Weather  Service,  who  has 
always  been  the  representative  of  the  U.  S.  Weather  Bureau  in  Balti- 
more, has  been  hitherto  designated  by  the  Chief  of  the  National  Service 
to  prepare  these  reports  and  has  had  access  not  only  to  the  United  States, 
but  to  the  State  records  in  this  worlv.  He  lias  been  able  to  employ  the 
services  of  a  trained  body  of  men  who  would  be  otherwise  entirely  beyond 
the  reach  of  the  State  for  such  an  investioation. 


Distribution  of  Plant  Life  in  the  State. 

The  distribution  of  animal  and  plant  life,  and  more  especially  of  the 
latter,  is  so  intimately  associated  with  the  physiographic  and  climatic 
conditions  that  prevail  that  the  Maryland  Weather  Service  has  under- 
taken a  Botanical  Survey  of  the  State  as  a  part  of  its  climatic  studies. 
For  the  past  three  years  several  trained  botanists  under  the  direction  of 
Dr.  Forrest  Shreve  have  been  engaged  in  the  different  sections  of  the 
state  in  making  a  detailed  investigation  of  the  botanical  conditions.  Not 
only  has  the  distribution  of  plant  life  been  found  to  be  dependent  on  the 
climate  and  physiography  of  the  state,  but  upon  the  agricultural  soils 
which  in  turn  find  their  ultimate  interpretation  in  the  underlying  rocks 
from  which  they  have  been  derived,  thus  bringing  the  work  of  the  State 
Geological  Survey  and  State  Weather  Service  into  close  association. 

The  botanical  survey  is  now  completed  and  a  report  is  at  the  present 
time  being  prepared,  which  will  be  issued  as  Volume  III  of  the  State 
Weather  Service. 


Survey  of  the  Swamp  Lands  of  the  State. 

A-  survey  of  the  swamp  lands  of  Maryland  has  been  made  in  connection 
with  the  topographic  survey  of  the  state,  in  which  the  State  Weather 
Service  has  participated  with  the  State  Geological  Survey  in  its  co-opera- 
tion with  the  Topographic  Branch  of  the  TJ.  S.  Geological  Survey.  This 
survey  has  shown  the  following  swamp  areas. 


24 


IXTRODUCTION 


Area  of  Swamp  Lands  in  the  Various  Counties  Computed  from  the 
Maryland  Geological  Survey  Maps. 

County.  Fresh.  Salt.  Total. 

Sq.  Mi.       Acres.  Sq.  >li.    Acres.         Sq.  Mi.  Acres. 

Baltimore     1.7  1,088  5.4  3,456  7.1  4,544 

Anne  Arundel    3.3  2,112  1.9  1,216  5.2  3,328 

Prince  George's   ...  8.6  5,504  0.2  128  8.8  5,632 

Charles    11.9  7,616  22.1  14,144  34.0  21,760 

Calvert    3.2  2,048  1.2  768  4.4  2,816 

St.  Mary's    0.3  192  1.3  832  1.6  1,024 

Harford     0.4  256  11.3  7,232  11.7  7,488 

Cecil     0.2  128  6.5  4,160  6.7  4,288 

Kent     0.4  256  7.9  5,056  8.3  5,312 

Queen  Anne's    9.7  6,208  4.5  2,880  14.2  9,088 

Talbot 0.3  192  5.3  3,392  5.6  3,584 

Caroline    9.7  6,208  2.6  1,664  12.3  7,872 

Dorchester    78.3  50,112  123.2  78.848  201.5  128.960 

Wicomico     17.1  10,944  22.1  14.144  39.2  25,088 

Somerset 7.7  4,928  68.5  43,480  76.2  48,768 

Worcester     33.0  21,120  35.4  22,656  68.4  43,776 

Garrett     4.5  2,880  4.5  2,880 

Other  counties   4.0  2,460  4.5  2,560 

Total    194.3       124,352  319.4       204,416         513.7         328,768 

it  will  thus  be  seen  that  the  State  of  Maryland  has  328,768  acres  of 
swamp  lands,  of  which  124,352  acres  are  fresh-water  swamps  and  204,416 
acres  salt-water  marshes.  The  eastern  and  southern  counties  of  the  state 
bordering  the  Chesapeake  Bay  and  the  Atlantic  Ocean  have  323,326 
acres,  of  which  118,912  acres  are  fresh  and  204,416  acres  are  salt.  The 
central  and  western  counties  have  5440  acres,  all  of  which  are  fresh. 

The  agi'icultural  soil  survey  of  Maryland,  which  is  being  carried  on  in 
co-operation  with  the  TJ.  S.  Bureau  of  Soils,  shows  a  considerably  larger 
acreage  of  swamp  lands  in  those  counties  surveyed  than  the  estimates 
above  given,  but  in  the  soil  survey  the  small  tracts  on  individual  farms 
were  computed,  while  the  topographic  maps  show  only  the  larger  areas, 
which  would  alone  be  considered  in  any  plan  of  government  reclamation. 
Counting  these  small  tracts  the  total  area  would  probably  reach  500,000 
acres. 

A  fuller  study  of  these  swamp  lands  is  now  in  progress  and  as  their 
present  condition  is  intimately  connected  with  the  climatic  conditions  of 


MARYLAND   WEATHER   SERVICE  25 

the  State  their  study,  in  part  at  least,  falls  within  the  province  of  the 
State  AVeather  Service. 

Other  Lixes  oe  Work. 

The  far  reaching  influence  of  climate  on  the  economic  and  social 
development  of  the  state  suggests  other  lines  of  investigation  that  require 
the  attention  of  the  State  Weather  Service. 

The  character  of  the  agricultural  soils,  although  fundamentally  deter- 
mined by  the  underlying  rocks,  is  also  to  no  small  degree  dependent  on 
the  physiography  and  climatic  features  of  the  State.  These  factors  must 
be  considered  in  any  comprehensive  study  of  the  agricultural  soils. 

The  health  of  any  community  is  also  to  no  inconsiderable  extent  de- 
pendent on  the  climate,  and  this  is  recognized  in  the  field  of  investigation 
known  as  medical  climatology.  In  Volume  I  the  present  writer  said  in 
discussing  this  subject  in  his  "  Plan  of  Operation  of  the  Service "'  that 
"  the  healthfulness  of  Maryland  as  a  place  of  residence  is  a  question  of  no 
small  importance  to  those  who  may  be  considering  the  advisability  of 
seeking  homes  in  our  midst,  and  actual  facts  should  be  presented  in  such 
a  manner  as  to  command  their  attention.  The  various  sections  of  the 
state,  their  marked  differences  in  temperature  and  rainfall,  may  be  shown 
to  be  adapted  to  the  physical  requirements  of  different  people,  and  it  is 
highly  important  that  these  facts  should  be  made  known. 

"  It  is  also  probable,  as  the  meteorological  records  over  considerable 
periods  are  carefully  studied,  that  some  districts  will  be  found  highly 
beneficial  to  people  suffering  from  certain  ailments.  It  is  the  purpose  of 
the  Maryland  AYeatlier  Service  to  have  some  expert  upon  medical  clima- 
tology carefully  study  its  records  and  prepare  a  report  upon  this  subject, 
and  already  arrangements  to  this  end  have  been  perfected." 

The  general  and  special  studies  earlier  enumerated  naturally  afford  the 
basis  for  a  considei-ation  of  tlie  crop  condition?  of  tlie  State  and  this 
subject  should  be  taken  up  in  a  comprehensive  way  as  the  data  collected 
become  adequate  to  the  discussion  of  so  great  a  subject.  The  agricul- 
tural products  of  the  State  far  surpass  those  in  every  other  line,  and  tlie 
State  Weatlier  Service  should  give  whatever  assistance  it  can  in  the  study 
of  the  important  problems  involved. 


26  INTfiODUCTIOJSr 

It  is  also  a  well  recognized  fact  that  the  character  and  distribution  of 
forest  growth  is  in  no  small  degree  determined  by  the  climatological 
features  which  have  already  been  described.  Since  forestry  studies  were 
organized  by  the  State  Geological  Survey  a  few  years  ago  a  State  Board 
of  Forestry  has  been  organized  and  the  investigation  of  our  forests  is 
now  well  under  way.  The  State  Weather  Service  can  aid  in  various  ways 
in  this  work. 

The  climate  in  its  various  relations  touches  human  life  in  so  many 
points  that  the  investigations  already  undertaken  and  proposed  will 
prove  not  only  of  great  interest,  but  of  greater  value  to  the  people  of 
the  state.  Eesults  of  real  worth  can  rarely  be  obtained  quickly,  but  the 
investigations  now  being  conducted  by  the  State  Weather  Service  are  of 
a  fundamental  character,  and  when  completed  will  cover  as  fully  as 
possible  the  jfield  of  climatology  in  its  various  relations  to  the  economic 
interests  of  the  State. 


REPORT  ON  THE 
CLIMATE  AND  WEATHER 

OF 

BALTIMORE  AND  VICINITY 

(Based  on    the   Observations   of   the  U.  S.   Weather   Bureau;  Supplemented    by  Obser- 
vations of  the  Maryland  State  Weather  Service,  and  the  U.  S. 
Army  Medical  Department.) 

PREPARED  BY  DIRECTION  OF 
WILLIS  L.  MOORE 

V 

Chief  oh    U.  S.  Wkather  Bureau 


BY 

OLIVER    LANARD    FASSIG 


THE  CLIMATE  OF  BALTIMORE 


I^siTEODUCTIOX 


For  more  than  thirty  years  the  United  States  Weather  Bureau  has 
maintained  a  station  of  the  first  order  in  Baltimore  City.  During  all 
these  years  the  weather  conditions  have  been  carefully  and  accurately 
noted  and  recorded  at  several  stated  hours  of  the  day  by  trained  observers. 
In  1893  the  instrumental  equipment  of  the  station  was  greatly  increased 
and  the  value  of  the  records  enhanced  by  the  acquisition  of  additional 
self-recording  instruments  by  means  of  which  a  continuous  record  has 
been  obtained  of  all  the  principal  elements  of  the  weather.  The  records 
of  the  Baltimore  station  now  show  the  local  state  of  the  atmosphere  dur- 
ing every  hour  of  the  day  and  night  since  1893,  barring  an  occasional 
brief  break  in  the  record  due  to  accidental  causes.  The  factors  thus 
continuously  noted  are  the  temperature  of  the  atmosphere,  the  pressure, 
rainfall,  sunshine,  wind  velocity,  wind  direotion  and,  since  1902,  the 
humidity.  This  mass  of  exceedingly  valuable  raw  material  for  the  study 
of  problems  in  local  climatology,  supplemented  by  an  almost  unbroken 
series  of  local  observations  made  since  1817  under  the  auspices  of  the 
United  States  Army  Medical  Department  and  the  Smithsonian  Insti- 
tution, has  never  before  been  subjected,  as  a  whole,  to  a  critical  analysis 
and  reduction.  It  is  evident  that  such  observations,  secured  at  enormous 
expense  of  time  and  money,  should  yield  benefits  beyond  their  immediate 
uses  at  the  time  of  recording,  however  valuable  these  may  be. 

The  weather  conditions  at  Baltimore  are  typical  of  conditions  within 
a  wide  area.  Allowing  for  small  differences  in  amplitude  of  variation 
due  to  local  surface  conditions,  an  analysis  of  the  Baltimore  observations 
may  with  safety  be  applied  to  much  of  that  portion  of  the  Middle  Atlantic 
States  lying  east  of  the  Appalachian  Mountains.  This  area  lies  about 
midway  between  the  rigorous  north  and  the  mild  south,  the  equable  ocean 
region  anrl  the  region  of  great  variability  in  the  interior  of  the  continent. 


30  THE    CLIMATE    OF    BALTIMORE 

Eainfall  is  abundant  and  quite  uniformly  distributed  throughout  the 
3^ear.  Storms  of  destructive  violence  are  of  rare  occurrence;  tornadoes 
are  almost  unknown.  The  season  of  safe  plant  growth  is  long,  and  sun- 
shine is  abundant. 

In  the  following  report  the  analysis  of  the  observations  is  divided  into 
two  distinct  parts.  The  first  part  deals  with  the  average  conditions  of 
the  atmosphere,  derived  from  many  years  of  statistical  data  relating 
to  temperature,  pressure,  humidity,  rainfall,  clouds  and  sunshine,  winds, 
etc.,  and  to  departures  from  their  normal  values.  In  brief,  it  deals  with 
the  climate  of  the  region  about  Baltimore.  The  second  part  is  devoted 
to  the  iveather,  or  actual  conditions  of  the  atmosphere  at  any  given  time 
as  regards  temperature,  humidity,  rainfall,  clouds,  wind — the  sum 
total  of  the  atmospheric  conditions.  Hence  weather  is  a  passing  phase  of 
climate.  Attention  will  be  directed  largely  to  storms,  cold  waves,  hot 
waves,  etc.,  as  well  as  to  the  gentler  phases  of  the  atmosphere  Avhich  con- 
stitute the  daily  routine  of  weather.  This  division  into  climate  and 
weather  is  necessarily  more  or  less  arbitrary,  and  the  lines  of  demarcation 
employed  by  different  writers  will  seldom  be  foimd  in  exactly  the  same 
places,  nor  will  the  strict  definition  be  consistently  adhered  to  in  the 
practical  treatment  of  the  subjects  by  the  same  writer.  But  the  division 
is,  in  the  main,  logical  and-  a  convenient  one  for  all  practical  purposes. 

Without  entering  unduly  into  details  regarding  the  plan  of  Part  I, 
attiention  may  be  directed  to  the  order  of  discussion  of  the  climatic  factors. 
As  far  as  possible  each  element  has  been  considered  with  reference,  (a)  to 
its  diurnal  period,  (b)  its  annual  period,  (c)  its  variability,  or  non- 
periodic  aspects  of  short  and  long  duration.  Tables  and  diagrams  have 
been  freely  employed,  the  statistical  tables  permitting  of  greater  accuracy 
of  statement,  the  graphic  method  affording  a  readier  means  of  presenting 
at  a  glance  the  salient  features  of  the  variability  of  the  climatic  elements 
from  hour  to  hour,  or  from  season  to  season. 

The  Geographical  Horizox  of  Baltimore. 

The  State  of  Maryland  is  situated  within  three  distinct  physiographic 
provinces.  The  low,  flat  Coastal  Plain,  averaging  about  60  feet  above 
mean  tide  and  cut  up  by  tidal  estuaries,  extends  from  the  Atlantic 
seaboard  westward  to  a  line  joining  Philadelphia.  Baltimore  and  Wash- 


MARYLAND    WEATHER    SERVICE  31 

ington.  where  it  is  separated  sharply  from  the  Piedmont  Plateau,  or 
middle  province.  The  Piedmont  Plateau  is  an  undulating  area  with 
elevations  rising  to  700  feet  or  800  feet,  and  extending  westward  to  the 
mountainous  and  high  plateau  region  of  the  Appalachian  Province.  The 
mountains  of  this  latter  province  form  a  system  of  parallel  ranges  extend- 
ing from  northeast  to  southwest  across  the  state,  rising  to  heights  of 
3000  feet.  The  city  of  Baltimore  is  partly  on  the  Piedmont  Plateau 
and  partly  on  the  Coastal  Plain.  The  country  to  the  north  and  west  is 
gently  undulating;  to  the  east  and  south  it  is  level  and  but  a  few  feet 
above  the  adjacent  estuaries  of  Chesapeake  Bay. 

ATMOSPHEEIC   PEESSUEE. 

As  a  direct  climatic  factor  the  pressure  of  the  atmosphere,  and  varia- 
tions in  this  pressure,  are  of  comparatively  minor  importance.  The 
effect  of  changes  in  the  height  of  the  barometer  upon  the  human  system 
does  not  begin  to  be  recognized  until  the  rise  or  fall  is  very  marked.  A 
diminished  pressure  causes  in  most  persons  a  feeling  of  lassitude  with 
increased  difficulty  of  breathing.  But  this  physiological  effect  is  not  ex- 
perienced, excepting  by  extremely  sensitive  persons,  until  the  barometer 
shows  a  fall  of  three  or  four  inches,  equivalent  to  an  ascent  of  three  or 
four  thousand  feet  above  sea-level.  The  extreme  variations  of  pressure 
at  any  one  place  do  not  often  exceed  an  inch  within  the  period  of  a  few 
days.  At  Baltimore  the  extreme  range  has  been  but  slightly  over  two 
inches  in  the  past  thirty-three  years.  The  change  in  the  pressure  ex- 
perienced during  the  passage  of  the  severest  type  of  cyclonic  disturb- 
ance is  less  than  the  permanent  diU'crcncc  in  pressure  between  the  east- 
ern and  the  higher  western  portions  of  the  State  of  Maryland;  and 
hence  less  than  is  experienced  by  travelers  daily  in  passing  from  Balti- 
more to  Pittsburg,  over  the  Alleghany  ^Mountains.  As  an  indirect  fac- 
tor, however,  in  the  climates  of  the  world,  and  as  a  direct  agency  in 
causing  movements  of  the  atmosphere,  the  pressure  changes  are  of  the 
highest  importance  and  take  rank  witli  those  of  temperature  and  rain- 
fall. 

In  anotlier  part  of  this  report,  the  more  general  relations  of  pressure 
will  be  discussed  in  connection  with  the  consideration  of  storm  move- 
ments.    Tlie  following  pages  are  devoted  mostly  to  the  local  conditions 


32 


THE    CLIMATE    OF    BALTIMORE 


and  variations  of  pressure  at  Baltimore,  based  upon  observations  made 
since  1871  under  the  auspices  of  the  United  States  Weather  Bureau. 
Observations  were  begun  in  Baltimore  on  January  1,  1871,  and  have 
been  maintained  in  an  unbroken  series  to  the  present  time.     Standard 


29.000  inches. 


TABLE  I.-MEAN  HOURLY  BAROMETRIC  PRESSURE. 

[In  inches  and  thousandths.] 

Local  time  is  6  minutes  slow. 


75th  mer.  time.  Jan.    Feb.  Mar.  Apr.  [  May  June  July  jAug.  Sept.    Oct.    Nov.  Dec.   Year 


1  A.M. 

2 

3 


.938 
.940 


.937 
.947 
.953 


8 .965 

9 j  .978 

10 I  .981 

11 I   .974 

Noon .953 

1 931 

.920 
.918 
.921 
.926 
.932 
.940 
.943 
.945 


10 945 

11 943 

Midnight 939 


Average. 


.885 
.883 
.880 

.881 
.886 
.894 
.906 
.912 
.911 
.906 
.891 
.869 
.855 

.a5i 

.851 

.857 
.866 
.875 
.878 
.883 
.885 
.883 
.881 


.943 


.881 


.892 
.888 
.883 
.883 
.89] 
.900 
.910 
.916 
.920 
.917 
.908 
.896 
.877 
.862 
.854 
.849 
.853 
.860 
.871 
.880 
.888 


.893 


.874 
.869 
.867 
.868 
.875 
.886 
.897 
.901 
.901 
.900 
.892 
.879 
.866 
.852 
.840 
.835 
.835 
.838 
.848 
.863 
.873 
.877 
.880 
.881 


.821 
.817 
.816 
.818 
.826 
.SSI 
.845 
.851 
.852 
.850 
.843 
.832 
.819 
.806 
.794 
.787 
.784 
.786 
.795 
.806 
.817 
.820 
.832 
.823 


.820 


.840 
.836 
.835 
.840 
.849 
.858 
.866 
.871 
.871 
.868 
.863 
.854 
.841 
.830 
.819 
.810 
.807 
.810 
.817 
.836 
.836 
.843 
.844 
.843 


.841 


.835 
.831 
.830 
.835 
.844 
.853 
.861 
.866 
.867 
.865 
.859 
.850 


.813 
.805 
.802 
.804 
.810 
.819 
.830 
.835 
.836 
.835 


.850 
.847 
.846 
.848 
.855 
.864 
.874 
.880 
.883 
.883 
.878 
.868 
.856 
.842 
.831 
.835 
.823 
.823 
.831 
.841 
.851 
.855 
.857 
.857 


.923 
.920 
.930 
.932 
.929 
.938 
.948 
.954 
.959 
.958 
.949 
.938 
.933 
.908 
.897 
.891 
.891 
.894 
.903 
.914 
.933 
.926 
.938 
.936 


.959 
.955 
.952 
.954 
.960 


.993 
.994 
.9fl3 
.985 
.969 
.951 
.937 
.931 
.929 
.933 
.939 
.946 
.954 
.957 
.961 
.960 
.958 


.959 
.9.59 
.958 
.959 
.964 
.970 
.981 
.991 
.995 
.993 
.981 
.963 
.944 
.934 
.933 
.9.36 
.941 
.9.50 
.957 
.964 
.965 
.966 
.965 
.963 


.966 
.967 
.967 
.964 
.963 
.968 
.978 
.990 
.996 
1.003 
.990 
.971 
.950 
.940 
.939 
.944 
.950 
.956 
.966 
.971 
.973 
.973 
.973 
.970, 


0.895 
0.893 
0.891 
0.893 
0.898 
0.906 
0.916 
0.934 
0.927 
0.937 
0.919 
0.905 
0.889 
0.876 
0.868 
0.865 
0.867 
0.873 
0.880 
0.888 
0.895 
0.898 
0.899 
0.897 


.853 


.924  i  .959  '  .962  I     .968   0.895 


Table  I  contains  the  average  hourly  values  of  the  station  pressure  at 
Baltimore  for  the  period  of  ten  years  from  1893  to  the  close  of  1902.  The 
values  are  derived  from  the  continuous  record  of  a  Richard  barograph,  cor- 
rected to  agree  with  personal  observations  of  a  mercurial  barometer  made 
daily  at  8  a.  m.  and  8  p.  m.  Each  mean  hourly  value  for  the  year  is  based  on 
over  3600  observations;  hence  these  values  may  be  regarded  as  very  close 
approximations  to  normal  averages  for  each  hour  of  the  day  for  the  year. 
The  station  elevation  has  been  123  feet  above  mean  tide  since  August  1,  1896. 
The  average  hourly  pressures  are  also  shown  graphically  in  Figs.  1  and  2. 


mercurial  barometers  were  read  at  stated  hours  from  two  to  five  times 
daily.  Since  1893  a  self-recording  barograph  has  furnished  a  continu- 
ous record  of  the  pressure  conditions  and  changes,  affording  excellent 
material  for  an  analysis  of  the  diurnal  fluctuations  of  the  barometer. 
The  rich  and  abundant  material  accumulated  by  the  Weather  Bureau 
during  the  past  thirty-three  years  has  been  reduced  and  di.scussed  with 


ilARYLAND    WEATHER    SERVICE 


33 


a  view  to  disclosing  the  nature  of  the  diurnal  and  annual  periodic 
changes  of  pressure,  as  well  as  the  irregular  and  secular  changes. 


Mdt.      3         6         9      Noon      3         6         9      Mot  Mot.      3         6         9      Noon      3         6         9      Mot. 

29.00+       In- 
.98  92 


i  1 

'      ■ 

1      ! 

' 

'  L/^ 

/     i    N 

'  I  1 

' 

1  ■   ;  1 

<C-*-Li 

\-.  j._^-_^. 

—  _f-^^**»N^- 

\~ , 

y^ ,  ,      .  "^ 

1,1 

\          Lt^ 

'  M 

1    *    j    ; 

!      >.^^     ' 

■  ,  , 

1  '  1  ' 

,    j    1    ■ 

' 

M  1  1 

!  ;  , 

:              ' 

' 

' 

-90  .84 


-n — '^ 

r        Arfl. 

1 

1            '    1 

'  '  1 

' 

_.^-*-i_ 

'     j 

:        1 

'       , 

:     i 

"\      "^"*^" 

V"^*"" 

i     i     ' 

rv  1    i 

i'     '    I    ' 

1         1     i     1 

1      iNi           1 

X    :        1    1 

I  T^T' 

1    1    , 

Mi 

1          1 

' 

Inches 
•  94 


2  9  90 


1   , 

n 

"^ 

"■ 

1    1    ' 

-^n 

1    '    1    '    1 

^ 

]   1 

1  1  M 

Year.  - 

' 

1 

i  i  ' 

1    ' 

1   ' 

1  ' 

1    1 

: 

1 

1   i      ' 

.    ,    1    I 

' 

! 

1 

!    1 

■    ' 

n   ;   ; 

'    ■    >    1 

TT- 

..,__. 

t      M  '  ' 

;        1    '    ' 

'  '  '  ■ 

1   1   i   '   ' 

'    ,       1 

MM 

1    '    1 

— -^  1    ;    ] 

^1.1 

1    i 

1 

!     ! 

1 

■    .^!    1 

1 

M    ' 

i 

:   1   1   : 

1  1  ; 

' 

■  '  'X' 

MM 

'  ' 

1    , 

Zl        '    ' 

i  M  \ 

1 

1 

'  '      M 

, 

^    '    '    ' 

.     ^ 

MM 

; 

' 

i 

./ 

1 

*■         1    i    1 

Ml. 

1 

' 

'   1 

1 

' 

\          '    ' 

^-^-^TH 

1  ^^^T- 

-T""'^ 

■  r^ 

\ 

f  i  l'^ 

C  '  I'M 

,        '        1 

!        !    j    I 

I'll 

\' 

Mil' 

1   1  j/f 

dix  1 

! 

\' 

1    ,  I 

J   '   1 

1 1 ' ' 

; 

'    ■    ;\.  ' 

r~ 

■    •    1 

1 1 1 

,  1  i  ' 

i 

1 

i  > 

; 

1  ■ 

1 

,    ^ 

ia-.  J-^ 

[ 

1 

1    1 

1  ■  i   j 

'        : 

1 

T* 

1 

' 

1    ' 

j 

i 

-n 

,    '    ■    ,    ! 

:   '   :   1   1 

1  1 

;    1 

j 

j       1 

t 

1 

1 

i  1  1  1 

pi:  -  ^  - 

'    ' 

: 

1 

t 

•    '    M 

1 

Inches 
.94- 


■86 


Mot.      3         6         9      Noon      3         6         9      Mdt.  mdt.      3         6         9      Noon      3         6         9      Mot. 

29.00r    In- 
•88         30.0 


j    ;    1    j    , 

1    1  --.„     1 

MI 

!     1     ' 

1     '     ■     ' 

! 

1 

!         I 

■ 

I 

1            y^ 

'           X 

.             ■              ' 

i-*''***^**- 

/^ 

'     1     . 

V  '  '  '  / 

'^'V^T 

.    ^ 

\^  y 

^NtO^' 

'    * 

' 

' 

' 

, 

! 

1 

t.  1    .    .    : 

'       1       ■              ! 

Fig.   1. — Hourly  Variatious  of  the  Barometer. 

Fig.  1.  The  (trcinue  height  of  the  barometer  at  each  hour  of  the  day  for  the  ten  years 
from  1892  to  190.S  is  shown  in  the  alwve  diagrams  for  the  months  of  January,  April,  July, 
Oct.,  and  for  the  entire  year.  The  height  of  the  column  of  mercury  which  the  pressure  of 
the  atmosphere  sustained  is  expressed  in  inches  and  hundredths  of  an  inch.  See  also  Fig.  2, 
and  Table  I. 


34  the  climate  of  baltimore 

The  Diurxal  Variations  of  the  Barometer. 

Within  the  tropics,  wliere  C3'clonic  storms  are  of  infrequent  occur- 
rence, the  diurnal  variation  of  the  barometer  is  the  most  marked  fea- 
ture of  the  barometric  curve.  So  regular  in  form  and  distinct  in  out- 
line are  these  changes  that  it  is  possible  by  inspection  of  the  curve  to  tell 
approximately  the  time  of  day.  The  amplitude  of  oscillation  near  the 
equator  is  about  one-eighth  of  an  inch.  This  amplitude  decreases  with 
distance  from  the  equator  but  is  still  recognizable  in  the  latitude  of  70 
degrees.  Along  the  parallel  of  Baltimore  the  amplitude  is  quite  marked 
in  a  curve  representing  average  hourly  changes  for  the  period  of  a  month 
or  more,  but  is  detected  only  by  the  experienced  eye  in  the  daily  curve, 
owing  to  the  relatively  large  irregular  changes  due  to  the  passage  of  the 
cyclonic  storms  of  the  middle  latitudes.  At  times,  especially  in  the 
summer  months  when  tropical  conditions  prevail  for  a  considerable 
period  in  our  latitudes,  the  diurnal  variation  is  very  distinct  for  days 
at  a  time.     (See  the  curve  for  August  7-13,  1900,  on  Plate  II.) 

THE    normal    DIURXAL    VARIATION    AT    BALTIMORE. 

The  mean  hourly  values  of  barometric  pressure  for  Baltimore  are  pre- 
sented in  Table  I  for  each  month  and  for  the  year.  The  results  for  each 
season  and  for  the  entire  year  are  also  shown  graphically  in  Fig.  1  on 
page  33  and  Fig.  2  on  page  36.  In  Table  II  the  same  values  are  ex- 
pressed in  terms  of  departures  from  the  average  value  for  the  entire  day. 
These  tables  and  diagrams  reveal  for  Baltimore  the  characteristic  double 
barometric  curve  so  well  known  to  the  meteorologist  from  the  results  of 
analyses  of  observations  in  all  parts  of  the  world,  with  perhaps  minor 
peculiarities  due  to  local  conditions.  The  fluctuations  are  well  marked 
in  all  months  of  the  year,  the  amplitude  varying  from  0.060  inch  in 
August  to  0.071  inch  in  March.  In  Fig.  2  the  distribution  of  pressure 
is  presented  by  a  method  not  frequently  employed  but  one  which  shows 
clearly  and  in  compact  form  the  successive  changes  from  hour  to  hour 
throughout  the  year.  Upon  a  system  of  coordinates  representing  the 
hours  of  the  day  and  the  months  of  the  year,  the  curved  lines  of  equal 
pressure  are  projected  in  such  manner  as  to  enable  one  to  find  the  exact 
pressure  at  any  hour  of  any  month.  These  curved  lines  are  sometimes 
called  "  isopleths.'^     For  example,  to  find  the  average  pressure  at  noon^ 


MARYLAND    WEATHER    SERVICE 


35 


in  Ajiril,  you  run  clown  the  vertical  line  marked  noon,  until  the  horizontal 
line  marked  April  is  intercei^ted,  and  find  the  isopleth  of  29.875.  This 
method  enables  ns  also  to  see  at  a  glance  the  chief  characteristics  of  the 
seasonal  distribution,  further  emphasized  by  differences  in  shading,  the 
lighter  shades  indicating  the  lower  pressures  of  the  warm  months  and  the 
darker  shades  the  higher  pressures  of  the  colder  months. 

TABLE  II.-HOUKLY  DEPARTCKES  FROM  MEAN  DAILY  PRESSURE. 
,In  thousandths  of  an  inchj 

Local  time  is  6  minutes  slow. 


75th  mer.  time.  Jan.    Feb.  Mar.  Apr.    May  June  July  Aug.  Sept.  Oct.    Nov.  Dec.   Year 


1  A.  M —.004 

2 -.005 

3 -.003- 

4 — .OOT- 

5 —.006 

.004 
.010 
.022 
.035 
.038 
.031 
.009 
-.012- 

—  .023  - 

—  .025- 

—  .022- 

5 —.017- 

6 -.011 


10.... 
11.... 
Noon 

1.... 

2 

3.... 

4.... 


7... 

8. 


—.003- 
.000 
.002 
.003 
.000 


Midnight   —.004 


.004 

.002 

-.001- 

-.002- 

.000 

.005 

.013 

.025 

.031 

.030 

.025 

.010 

-.012- 

-.026  - 

-.030- 

-.030- 

-.034- 

-.015- 

-.006- 

■.<m- 

.004! 
.002! 
.000, 


.006 

.002 

-.003 

-.003 

.005 

.014 

.024 

.030 

.034 

.031: 

.022i 

.010: 

-.009 

-.024 

-.032 

-.037i 

■.ft33 

-.026 

-.015 

-.OOfJ 

.CK)2 

.006 

.006 

.007 


.003 

.002- 

.004 

.003- 

.004; 

.015 

.026 

.030' 

.0:30 

.029 

.021; 

.OOS 

.005- 

.019- 

.031- 

.036- 

xm- 

.033- 
.023  - 
.008- 
.001  - 
.0061 
.009| 
.010; 


.001 
.003 
.004 
.003 
.006 
.017 
.025 
.031 
.032 
.030 
.023 
.012 
.001 
.014 
.026 
.033 
.036 
.034 
.025 
.014 
.003 
.000 
.002 
.002 


.001 
-.005 
.006 
.001 
.008 
.01 
.025 
.030 
.030 
.02 
.023 
.013 
.000 
.011 
.022 
.031 
.034 

.o;ii 

.(V24 
.015' 

.005!- 

.001 

.003 
.002 


.000- 

-.OftS 

-.001' 

-.004- 

.(K)6 

-.004 

-.005- 

.(K)7 

-.004 

.000- 

.005 

-.003- 

.009 

.(K»3 

.005 

.017; 

.011 

.014 

.026 

.021 

.034' 

.0311 

.(t'7 

.OiO. 

.032, 

.030 

.oa5: 

.o;30, 

.03(1 

.034 

.024! 

.035 

.035 

.0151 

.015 

.OI4I 

.003! 

.(K)3 

—  .003  - 

.010- 

.011 

—  .016  - 

.023- 

.023 

-.037  - 

.030- 

(138 

-.0:33- 

.033- 

0:^0 

-.0:33- 

.031  - 

.030 

—  .0:30'- 

.025- 

.(t'3 

-.031:- 

.016- 

.(tl3 

-.010- 

.005  — 

.003 

-.001  - 

.000 

Am 

.003 

.(Kin 

.004 

.004 

.OOOj 

.004 

.003- 

.000 -.003 
-.004— .003 
-.007 -.004 
-.005— .003' 

.001      .002 


.008 
.020 
.034 
.0:35 
.033 
.026 
.010 
-.008- 


.008 
.019 
.029 
.033 
.031 
.019 
.001 
-.018 


-.033— .038  ■ 
-.028— .030  ■ 
-.030— .036 
-.037— . 031- 
-.030— .012 - 
-.013— .(K)5- 


.(K15 

.002 

.(H)2 

.C03 

.(K)2 

.004 

.001 

.003 

.001 

.001 

-.00-. 
-.001 
-.001 
-.(H)4 
-.005 
.000 
.010 
.032 
.028 

.0:35 

.022 
.003 
-.018 
-.038 
-.029 
-.024 
-.018 
-.012 

-.m 

.003 
.004 
.005 
.005 
.002 


.000 

.003 

-.004 

.003 

.003 

.011 

.021 

.029 

.032 

.033 

.034 

.010 

-.006 

-.019 

-.037 

.030 

-.028 

-.033 

015 

-.007 

.000 

.003 

.004 

.003 


Table  II  contains  the  average  hourly  departures  from  the  normal  monthly 
status  pressures  recorded  in  Table  I,  the  values  being  expressed  in  thou- 
sandths of  an  inch  of  pressure.  Figures  preceded  by  the  minus  sign  represent 
values  below  the  normal  for  the  day;  those  without  sign  represent  values 
above  the  normal.  For  example,  examining  the  column  headed  "  year "  we 
observe  that:  the  atmospheric  pressure  is  at  or  below  the  average  for  the  day 
from  1  a.  m.  to  4  a.  m.,  above  the  average  from  5  a.  m.  to  noon,  falls  below 
the  mean  again  at  1  p.  m.,  remaining  below  until  8  p.  m.,  then  again  exceed- 
ing the  mean  to  midnight;  that  the  average  time  of  the  primary  maximum 
for  the  day  occurs  between  9  a.  m.  and  10  a.  m.,  and  the  primary  minimum  at 
4  p.  m.;  that  a  secondary  maximum  occurs  at  11  p.  m.,  and  a  secondary 
minimum  occurs  at  3  a.  m.  The  maximum  and  minimum  phases  vary  from 
month  to  month  as  shown  in  Table  III  and  in  Fig  3. 


PHASES    OF    THE    DIURNAL    OSCILLATION. 

The  four  principal  phases  are  distinctly  revealed  in  all  of  the  normal 
curves   (see  Figs.   1,  2  and  -1).     The  primary,  or  morning,   ma.ximum 


36 


THE   CLIMATE    OF    BALTIMORE 


occurs  about  9.15  a.  m.,  local  time,  on  the  average  during  the  year.  The 
time  varies  with  the  season.  During  the  winter  months  the  crest  of  the 
maximum  wave  occurs  about  half  an  hour  later  and  during  the  summer 
months  half  an  hour  earlier,  making  a  difference  of  about  an  hour  be- 
tween the  earliest  and  latest  average  appearance.  The  most  variable  in 
time  of  appearance  is  the  primary,  or  afternoon,  minimum.  From 
January,  when  this  phase  occurs  about  3  p.  m.,  there  is  a  steady  retard- 
ation in  the  time  of  occurrence  of  the  minimum  to  about  5  p.  m.  in 
May,  where  it  remains  until  August,  when  the  time  again  moves  for- 
ward to  the  winter  minimum  at  3  p.  m.     The  average  occurrence  of  the 


9         '0        11      NOON      12345678910 


opleths  of  Hourly  Pressure. 


Fig.  2  shows  the  average  distribution  of  atmospheric  pressure  throughout  the  day  and 
year,  based  on  observations  of  ten  years  of  hourly  readings  of  the  barograph.  The  upper 
marginal  figures  indicate  the  hour  of  the  day,  the  marginal  letters  indicate  the  months  of  the 
year.  The  line  enclosing  the  area  of  lightest  shading  defines  the  time  of  day  and  month 
"when  the  barometer  is  normally  lowest ;  increase  in  the  intensity  of  shaded  areas  shows  an 
increase  In  the  height  of  the  barometer.  The  diagram  shows  that  the  barometer  is  lowest 
at  about  5  p.  m.  in  the  month  of  May ;  that  it  is  highest  at  10  a.  m.  in  the  month  of  December. 
The  curved  lines  show  the  hours  of  the  day  and  the  months  of  the  year  when  the  barometer 
readings  are,  on  the  average,  equal;  these  lines  are  called  chrono  isobars,  or  isopleths  of 
pressure.  The  dotted  lines  marked  S.R.  and  S.S.  show  the  time  of  sunrise  and  sunset.  See 
also  Table  I. 


minimum  for  the  year  is  about  4.15  p.  m.,  local  time.  The  secondary, 
or  night,  maximum  occurs  about  11  p.  m.,  the  time  of  occurrence  being 
fairly  constant  but  varying  from  10  p.  m.  in  December  to  11.30  p.  m. 
in  the  summer  months.  The  secondary,  or  night,  minimum  occurs 
quite  uniformly  at  3  a.  m.  from  May  to  December,  with  extreme  limits 


MARYLAXD    WEATHER    SERVICE 


of  3.30  a.  m.  in  January  and  February  and  2.30  a.  m.  in  April.  The 
most  marked  feature  of  the  variations  in  time  of  occurrence  of  the  dif- 
ferent phases  is  the  gradual  increase  in  the  time  interval  between  the 


TABLE  III.— PRESSURE  PHASES. 
(75th  meridian  time.— Local  time  is  6  minutes  slow. 


January 1 

February  I 

March ] 

April 

May 

June 

July 

Aug'uet 

Sejjtember 

October 1 

November I  2 

December 


Morning- 
maximum. 


8  9  10 


6..i 

6  2 
4  5 

7  3 
6  4 


Sums . 


Annual  average 


9:30 
9:30 
9:00 
9:00 
8:30 
8:30 
9:00 
9:30 
9:00 
9:00 
9:00 
10:00 


210  3    9:0 


Afternoon 
minimum. 


p.m. 


2  3 


533 


4  5 


4 
8 
9 
4 

3^ 

7  1 


..  3:00 

..  3:00 

..  4:00 

. .  4:30 

2  5:00 
..  5:00 

3  5:00 
5  5:30 
1  4:30 

..  4:00 

..  3:00 

..  3:00 


354011 


4  6  1    4:08 


Night 
maximum. 


8  9  101112 


I    I 


■iViV 


4  8294327     17 


>  ft 
< 


11:00 
10:30 
IhOO 
11:30 
11:30 
11:00 
11:30 
IIMO 
11:0U 
11:00 
10:00 
10:00 


10:55 


Night 
minimum. 


Mid- 
n't. 


12      12  3  4 


6|  2. 
1. 
3. 

5  2. 

1..I 


1    1129582915 


1  9  3!  1 


3:30 
3:30 
3:00 
2:30 
3:00 
3:00 
3:00 
3:00 
3:00 
3:00 
3:00 
3:00 


3:05 


Table  III  shows  the  average  hour  of  occurrence  of  the  daily  maximum  and 
minimum  station  pressure  for  each  month  and  for  the  year  during  a  period 
of  ten  years,  and  also  the  extent  to  which  the  times  of  occurrence  of  these 
phases  have  varied  from  the  average  time.  For  example,  examining  the 
afternoon  minimum  phase,  the  most  variable  of  all,  we  find  that  in  10  years 
it  occurred  in  January  7  times  at  3  p.  m.,  4  times  at  4  p.  m.,  and  once  at  5 
p.  m.:  and  that  the  average  time  of  occurrence  for  the  year  is  about  3  p.  m.; 
that  in  August  it  occurred  at  5  p.  m.  6  times  and  at  6  p.  m.  5  times;  and  as 
an  average  time  we  have  about  5.30  p.  m.,  etc.  The  average  values  are  also 
shown  graphically  in  Fig.  3. 


primary  maximum  and  primary  minimum  with  the  approach  of  sum- 
mer, from  five  hours  and  a  half  in  the  winter  months  to  eight  hours 
and  a  half  in  May  and  June.  This  increasing  interval  is  due  mostly  to 
variations  in  the  time  of  occurrence  of  the  primary  minimum.  These 
phases  are  shown  in  detail  in  the  following  table  and  in  Fig.  3. 


38 


THE    CLIMATE    OF    BALTIMORE 


INTERVALS  BETWEEN'  PRINCIPAL   PHASES  OK   PRESSURE. 

(Expressed  in  Hours  and  Minutes.) 


Intervals 
between. 

Jan. 

Feb. 

Mar. 

Apr. 

May  June 

i 
July  Aug. [Sept. 

Oct. 

Nov. 

Dec. 

Year 

A.  Primary 
max.  and  min. 

B.  Secondary 
max.  and  min. 

5:30 
4:30 

5:30 
5:00 

7:00 
4:00 

7:30 
3:00 

8:30 
3:30 

8:30 
4:00 

8:00 
3:30 

8:00 
3:30 

7:30 
4:00 

7:00 
4:00 

6:00 
5:00 

5:00 
5:00 

7:00 
4:05 

FMAMJJASOND 


■ 

'~ 

ill:         '     '  ,        1 

-- 

■^ 

:    1  '  1  Mi  ,  1 

V 

1 

s 

> 

1    1        '    ; 

\ 

/ 

s. 

-* 

y 

^ 

;a 

'  [ 

^^ 

_ 

> 

S 

1 

v, 

k-j 

^ 

■" 

V 

l^ 

'V 

rf 



_. 

^ 

, 

p 

S 

3PM 

1  ^ 

/ 

s 

1^ 

r"~ 

1 

h-j  '  '  ' 

• 

1  1 1 

A 

9  A.  M.  - 

i; 

L 

_ 

. 

.. 

t^'j 

. 

^-1 

> 

SJ 

,<" 

-? 

"n 

6A    M.- 

fr/ 

1 

3AM- 

" 

N 

! 

V 

r' 

i  1 

^  i 

'    i    1 

r  :  I 

3  A.  M. 


9P.M. 


3  P.  M. 


-    9A.M. 


6  A.  M. 


Fig.  3. —  Principal  Phases  of  Diurnal  Oscillation  of  Pressure. 

Fig.  3  indicates  the  time  of  occurrence  of  the  maximum  and  minimum  points  reached  by 
the  barometer  in  the  diurnal  oscillation.  The  upper  marginal  letters  represent  the  months 
of  the  year ;  the  figures  indicate  the  hours  of  the  day.  The  curved  lines  represent  i-espec- 
tively  (a)  the  time  of  occurrence  of  the  secondary  maximum;  {h)  the  primary  minimum  ; 
(c)  the  primary  maximum ;  (d)  the  secondary  minimum.    See  also  Table  III. 


MARYLAND    WEATHER    SERVICE 


39 


AMPLITUDE  OF  OSCILLATIOX. 

(In  Thousandths  of  an  Inch.) 


Jan.    Feb. .  Mar.  I  Apr. ,  May   June 


A.  Diurnal 

Amplitude.      63         61         71         66 

B.  Nocturnal 

Amplitude.       9  6        10        14 


ft*         64 
6  9 


July,  Aug.  Sept.'  Oct.  |  Nov.  Dec. 


60         68         65         63         64 

1 

11  8    1      9  8         10 


Tear 


63 


TABLE  IV.-HOL'RLY  VARIATIONS  OF  PRESSURE  ON  CLEAR  AND  ON 
CLOUDY   DAYS. 

(Expressed  In  thousandths  of  an  inch  as  departures  from  the  daily  average.) 


Clear  days  in 

To'al 

(90  days 

Departure. 

Cloudy  days  in 

Total 

(90  days) 

Departure. 

75th  Meridian 
Time. 

Jan.  and 

Feb. 

(60  days) 

Departure. 

July 

(.30  davs) 

Departure. 

Winter 

(60  days) 

Departure. 

Summer 

(30  days) 

Departure. 

Midnight  .   ... 

1 

2 

3 

4 

5 

6 

-.053 
-.037 
-.029 
-.021 
-.018 
-.005 
-.004 
+  .022 
+  .041 
-.056 
+  .063 
-r.058 
-r.040 
+  .019 
+  .003 
-.008 
-.013 

—  .014 
-.014 
-.012 
-.015 

—  .019 
-.026 
-.036 
-.051 

-.006 
-.006 
-.007 
-.008  • 
-.002 
+  .009 
+  .018 
+  .028 
+  .034 
+  .0.34 
+  .a32 
+  .028 
+  .023 
+  .009 
-.004 
-.017 
-.026 
-.0.38 
-.035 

-.0:31 

-.025 
-.016 
-.013 
-.008 
-.006 

-.028 

—  .022 
-.018 
-.014 
-.010 
+  .002 
+  .011 
+  .025 
+  .038 
+  .045 
+  .048 
+  .043 
+  .032 
+  .014 

.000 

—  .013 

—  .020 
-.024 

—  .024 
-.022 

—  .020 
-.018 
-.020 
-.022 
-.028 

-.003 
+  .010 
+  .013 
+  .012 
+  .006 
-^.006 
-.008 
+  .015 

-.001 
-.004 
-.011 
-.014 
-.011 
-.004 
+  .011 
+  .023 

.000 
+  .003 
+  .001 
-.001 
—.002 
+  .001 
+  .010 

+  .019 

8  

-.024                +.029 

4-.  026 

9 

10 

11 

Noon   

1 

2 

3 

4 

5 

6 

+  .032        i        +.034        1        +.033 
-+-.034                +.030                +.033 
+  .024                 +.027                 -J-.026 
+  .U04                +.020        ,        -.012 
-.016                +.005                -.006 
-.025                -.003               -.014 
—.027               —.011                -.019 
—.027               -.017                -.022 
-.023                -.020                -.022 
-.025               -.017               -.021 
—.011                —.013               —.013 

8 

—  .006                —.004                —.005 

9 

—  .003                +.001                 —.001 

10 

11 

Midnight 

-.003                -^.003                   .000 
+  .003                +.001                +.002 
+  .001                +.001                   .000 

Table  IV  shows  the  amount  of  the  diurnal  variation  of  pressure  on  clear 
days  as  compared  with  cloudy  days  in  order  to  detect  any  difference  due 
to  cloudiness.  For  this  purpose  CO  clear  days  in  January  and  February  and 
?,0  clear  days  in  July  were  chosen,  to  be  compared  with  CO  cloudy  days  in 
January  and  Feliruary  and  HO  cloudy  days  in  July.  The  effect  of  irregular 
\ariations  of  the  barometer  due  to  the  passage  of  storms  was  first  eliminated 
from  the  actual  means.     The  results  are  also  graphically  shown  in  Fig.  4. 

In  individual  months  the  time  of  occurrence  of  these  phases  varies 
considerably  from  the  average  times  for  the  entire  ten  years,  as  may  be 
seen  in  Table  III,  which  shows  the  frequency  of  occurrence  of  the  differ- 
ent phases  for  each  month  for  the  ten -year  period. 


40 


THE    CLIMATE    OF    BALTIMORE 


DIURNAL  VARIATIONS    OF    PRESSURE   ON    CLEAR   AND    CLOUDY    DAYS. 

In  Tables  I  and  II  and  Figs.  1  and  2  the  average  distribution  of 
pressure  is  shown  for  all  conditions  of  the  weather  during  a  period  of 
ten  years.  In  order  to  determine  the  effect,  if  any,  of  cloudiness  upon 
the  oscillation  of  pressure,  selection  was  made  of  60  clear  days  in  Janu- 
ary and  February  and  30  in  July  to  be  compared  with  a  like  number  of 


MDT 
+  .05 


9  Noon 


-.0  5 
••-.05 


-.05 
♦.05 


y""             ^^ 

7                   \ 

j/ .''                   ■*            X, 

•'  — ~  -  .                 ^'^                        ^        S 

j'     ■  _i,_.j_^_:^Jl2_. -,,_.,.-,__,.-  —  .».—  -^«»-.  --!.--.  .ii.  J."  *.  ir    ,^    f'> 

'-         "     ^^^                           "^       ""-^  XL  ^■^j^r'^X- 

xS                                                   ^"^i 

/                                                                                 s 

i           I 

"""  '    :    .^sf--^     :"  "'-^-^ 

Tfy'  ;                           ..^>s^ ,„    .      v.--^-^-v^ 

~'--+'                                                 "*?.-'—•,_ "'^  ^  — -"''^    1 

>^            ^jfyf"^ 

"*-._  --"'' 

'          *" "    ~'^5»                                                 1 

>;:,  — H^  ^> 

^^P                 1      "'nj^         V 

-..  .               .>^                                           >        S                                  <               ^. ..-.., 

1         '•i-jfj;^                            ■'    "■"  -■  ■      ■    -  TT--  '-S^  '        -1-             \    i     ,-r-- 

\  \  ^  1    ^"Hn                                               '  •!,  "«.._        1     '•'f\  -t— — 1        1 

-j-^                                              r  "t--sii-+«-— T            "*■-«, 

1                      .                                                                                                        1 

i                                           i                      '    1                                                                 i 

-.05 
ud5 


0        /) 


-.05 
+.05 


Fig.  4. — Diurnal  Variations  of  Pressure  on  Clear  and  on  Cloudy  Days. 
Clear.  Cloudy. 

Fig.  4  shows  the  hourly  chanKes  of  the  barometer  on  selected  da3'S  approximately  similar 
in  all  respects  excepting  as  to  tlie  amount  of  cloudiness.  The  cootinuous  lines  show  the 
movements  of  the  barometer  on  clear  days  and  the  dotted  lines  on  cloudy  days,  for  (a)  win- 
ter months,  Q>)  summer  months,  and  (c)  for  the  year.  The  hourly  heights  of  the  barometer 
are  expressed  in  hundredths  of  an  inch,  as  departures  from  the  average  height  for  the  entire 
day.    See  also  Table  IV. 

cloudy  days  in  the  same  months,  as  far  as  possible.  The  computed 
variations  are  shown  in  tabular  form  in  Table  IV,  and  graphically  in 
Fig.  4,  on  page  40,  after  first  eliminating  the  effect  of  irregular  fluc- 
tuations of  the  barometer  due  to  passing  cyclonic  disturbances.  The 
primary  maximum  and  minimum  phases  differ  but  little  from  those  of 


MARYLAND    WEATHER    SERVICE  41 

the  normal  curve,  although  the  amplitude  on  clear  days  is  somewhat 
exaggerated,  especially  so  during  the  winter  months.  The  curves  for 
the  normal  and  for  the  cloudy  days  coincide  very  closely  for  the  sum- 
mer months  and  for  the  year,  but  diverge  in  the  early  morning  hours 
of  the  winter  months.  The  most  striking  feature  of  the  curve  for 
totally  clear  days  is  the  wide  divergence  from  the  normal  curve  in  win- 
ter during  the  night  and  early  morning  hours. 

THE    DIURXAL    BAROMETOIC    WAVE. 

The  diurnal  variations  of  the  barometer  described  in  the  preceding 
paragraphs  are  not  simply  of  local  occurrence  but  are  part  of  a  general 
phenomenon  extending  over  the  greater  portion  of  the  earth's  surface. 
The  maximum  and  minimum  phases  pointed  out  occur  in  all  localities 
at  approximately  the  same  hours  of  local  time.  As  stated  above,  this 
pressure  wave,  as  it  may  be  called,  has  its  greatest  development  in  or 
near  the  equatorial  belt,  and  diminishes  in  amplitude  with  distance 
north  and  south  of  the  equator.  It  has  some  resemblance  to  a  double 
atmospheric  wave  passing  completely  round  the  earth  from  east  to  west 
every  twenty-four  hours,  having  a  velocity  at  the  equator  of  about  one 
thousand  miles  per  hour.  By  plotting  upon  a  map  of  tlic  world  the 
departures  from  the  normal  daily  pressure  for  successive  hours  of  the 
day  at  a  large  number  of  stations  uniformly  distributed  over  the  north- 
ern and  southern  hemispheres,  and  joining  such  stations  as  have  equal 
departures  of  pressure  for  the  same  hour,  we  have  presented  to  us  four 
systems  of  pressure-distribution,  consisting  of  two  areas  of  low  pressure 
and  two  areas  of  high  pressure.  These  systems  completely  encircle  the 
globe  and  closely  resemble  in  form  the  cyclonic  and  anticyclonic  systems 
of  the  middle  latitudes,  but  differ  from  them,  among  other  things,  in 
covering  an  area  vastly  greater,  and  in  moving  in  the  opposite  direction. 
The  diurnal  fluctuations  of  the  barometer  are  the  local  evidence  of  this 
vast  double  atmospheric  wave  passing  round  the  globe  daily.  The  west- 
ward propagation  of  these  waves  near  the  equator  is  represented  in  Fig.  5 
on  page  42 ;  the  curve  shows  the  time  of  occurrence  of  the  different 
phases  of  the  double  wave,  its  amplitude,  and  the  direction  of  propaga- 
tion along  the  path  of  greatest  development.  The  character  of  these 
waves  is  further  indicated  in  fhe  diagrams  of  Plato  T,  in  which  the  succes- 


42 


THE    CLIMATE    OF    BALTIMORE 


sive  areas  of  high  and  low  pressure  are  exhibited  at  intervals  of  two 
hours  in  passing  from  east  to  west  across  the  North  and  South  American 
continents/ 

This  double  atmospheric  wave,  or  tide,  is  so  intimately  associated 
with  the  apparent  diurnal  movements  of  the  sun  that  the  conclusion  is 
almost  irresistible  that  the  pressure  changes  are  due  primarily  to  changes 
of  temperature.  This  relationship  has  not  yet  been  satisfactorily  demon- 
strated to  be  that  of  direct  cause  and  effect,  but  there  seems  to  be  a  gen- 
eral consensus  of  opinion  that  the  primary  maximum  and  the  primary 
minimum  phases  of  pressure  are  direct  effects  of  the  sun's  heat.     The 


N. 

t 

om             3am           M^dn               9 

pm 

,       6pr, 

3om           Noon                  9am               t 

am 

*040 
t020 

fO-IO 
tO?0 

/ 

\ 

/ 

\ 

/ 

\ 

W.o 

-020 
-040 

/ 

\ 

oE 

-0?0 

-040 

\       ^ 

/ 

V 

/ 

/ 

V 

y 

s. 

"""1 

Fig.  5. — The  Diurnal  Barometric  Wave. 

Fi^.  5  shows  the  direction  of  movement  of  the  diurnal  barometric  wave,  from  east  to  west 
around  the  globe  ;  also  the  local  time  at  which  the  crests  and  the  hollows  of  the  wave  pass 
over  any  locality  along  the  path  of  the  greatest  development  of  the  wave,  near  the  equator. 
The  extent  of  the  diurnal  rise  and  fall  of  the  barometer  is  shown  by  the  figures  to  the  right 
and  left  of  the  diagram,  which  express  the  departures  above  and  below  the  normal  height 
for  the  day,  in  thousandths  of  an  inch  of  mercury.    See  also  Plate  I. 

theory  advanced  many  years  ago  to  account  for  the  chief  maximum  and 
minimum  phases  seems  plausible.  At  the  time  of  day,  between  9  a.  m. 
and  10  a.  m.,  when  the  atmosphere  is  being  warmed  most  rapidly  and 
the  tendency  of  the  air  to  rise  in  consequence  is  greatest,  the  upper  and 
colder  layers  impede  this  upward  movment,  resulting  in  a  temporarily 
increased  tension  at  the  surface  of  the  earth.  When  this  tension  is  re- 
lieved the  barometer  begins  to  fall,  reaching  its  lowest  point  about  the 


'Fassig,  O.  L.     The  Daily  Barometric  Wave.     BulL  No.  31,   U.  S.  Weather 
Bureau.     8°.     Washington.  D.  C,  1902,  pp.  62-65,  12  pis. 


rHK    DHUNAI.    IIAKIIMETKIC    W  A\  !•:. 
(75T]1     MERIDIAN    TIMK.) 

?:in  pressure  of  tin-  day  arc  expressed  in  Ihonsaiidtlis  of  an  inch  of  mercury.) 


MARYLAXD    WEATHER    SERVICE  43 

middle  of  the  afternoon  when  the  upward  movement  of  the  warm  air 
may  be  assumed  to  be  least  impeded.  In  this  connection,  Fig.  17,  on 
page  76,  is  significant,  showing  the  average  hourly  rate  of  change  of 
temperature  for  the  year,  compared  with  the  curve  representing  the 
average  hourly  variation  of  pressure  for  the  year. 

As  has  already  been  stated  above,  the  pressure-wave  attains  its  greatest 
amplitude  in  the  equatorial  belt  where  the  diurnal  temperature  changes 
are  greatest,  and  over  the  continental  masses  north  and  south  of  the 
equator  where  the  diurnal  range  of  temperature  is  most  marked.  (See 
Plate  I.) 

According  to  Dr.  Hann,^  in  seeking  an  explanation  of  the  diurnal 
variations  of  the  barometer:  "We  had  better  deal  with  the  action  of 
the  sun  on  the  upper  strata  of  the  atmosphere  and  treat  this  as  the 
principal  cause.  The  actinometrical  observations  show  us  that  these 
upper  strata  absorb  a  considerable  amount  of  heat.  The  diurnal  heat- 
ing action  of  the  sun  on  the  upper  strata  would  harmonize  far  better 
with  the  general  uniformity  of  the  daily  barometric  oscillation  along  the 
different  parallels  of  latitude  as  well  as  with  its  general  independence 
of  weather.  We  need  not  quite  exclude  local  influences,  but  these  seem 
to  be  more  of  a  secondary  character."  This  view  is  also  held  by  Lord 
Kelvin,  who  seems  to  have  been  the  first  to  suggest  this  explanation. 

CORRECTIONS    FOR    REDUCTION    TO    TRUE    MEAN    PRESSURE. 

The  determination  of  the  daily  mean  barometric  pressure  based  on 
24-hourly  observations  for  ten  years  enables  us  to  apply  the  necessary 
corrections  to  any  given  combination  of  daily  observations  in  order  to 
obtain  a  true  mean.  The  following  table  contains  the  corrections  for 
each  month  and  for  the  year  which  must  be  applied  to  the  series  of 
observations  made  according  to  any  of  the  five  systems  most  frequently 
employed  in  barometric  observations  in  this  country.  The  average  of 
the  three  observations  made  at  7  a.  m.,  2  p.  m.,  and  9  p.  m.  approaches 
most  nearly  the  true  24-hourly  mean  for  the  day,  when  the  9  p.  m.  ob- 
servation is  given  double  weight. 

'Hann,  J.    The  Theory  of  the  Daily  Barometric  Oscillation.    Quart.  Journ. 
Roy.  Met.  Soc,  London,  1899,  p.  40. 
4 


44 


THE    CLIMATE    OF    BALTIMORE 


CORRECTIONS   FOR   DIURNAL    VARIATIONS  OF  THE   BAROMETER. 

(In  Thousandths  of  an  Inch.) 


Hours  of 
observation. 


7A.+2P.+9P. 


7A.+2P.-*-2(9P.) 


7A.+3P.+11P. 


10  A.+IO  P. 


8A.+8P. 


Jan. 

Eeb. 

Mar. 

Apr. 

May 

June 

+4 

+4 

-1 

-3 

-3 

-3 

+2 

+2 

—1 

—2 

-1 

-1 

+5 

+5 

+1 

-1 

0 

-2 

-20 

-17 

-18 

-18 

-15 

-14 

-11 

-11 

-12 

-11 

-8 

-8 

July 

Aug. 

Sept. 

Oct, 

Nov. 

Dec. 

Year 

-4 

-3 

-2 

+1 

+2 

+5 

-1 

2 

—2 

-2 

+2 

+1 

+2 

0 

-2 

-1 

0 

+2 

+3 

+5 

+1 

—15 

-16 

-18 

-18 

-18 

-20 

-18 

—1 

-8 

—10 

-14 

-16 

-12 

-11 

The  Annual  March  of  Atmospheric  Pressure. 
In  order  to  determine  the  changes  of  pressure  from  day  to  day  during 
the  course  of  the  year,  the  daily  averages  of  the  Baltimore  observations 
covering  a  period  of  30  years  were  reduced  to  what  may  be  called  normal 
values  for  each  day  of  the  year.  In  obtaining  these  normals  the  sea-level 
values  were  employed,  but  corrections  for  diurnal  variation  were  not 
applied.  The  results  are  shown  in  Table  Y  on  page  45  and  graphically 
by  means  of  curve  (d)  of  Plate  III.  This  curve  shows  a  fall  in  pres- 
sure from  month  to  month  from  January  to  May.  During  May,  June 
and  July  the  pressure  remains  fairly  uniform,  followed  by  a  compara- 
tively rapid  rise  in  August  and  September.  During  October  there  is  a 
slight  fall  followed  by  a  rise  to  January.  The  curve  of  daily  changes 
does  not,  however,  show  a  steady  rise  and  fall  from  season  to  season. 


J 

F 

[V 

A 

^ 

J 

J 

A 

c 

c 

N 

D 

1 

1     1 

IncTies 

;     1 

1     1 

1 

i 

1   I 

1 

! 

i 

,  I 

1 

30.00 

1    ! 

1  I 

1 

~>i 

-f 

,  1 

P-*- 

"V 

1     ! 

-<■ 

r 

'V, 

j     1 

;  y 

" 

' — 

•* 

! 

.92 

\ 

1     I 

/ 

I 

> 

/ 

s 

/ 

"i 

r- 

-n 

^ 

'~-1 

^ 

29.84 

1 

( 

— 

1 

'     ! 

i 

t  1 

- 

i 

1 

! 

1 

.J, 

.. 

i 

i 

_ 

1  1 

i  1 

_ 

_J 

_ 

_^ 

_ 

_ 

^ 

29.84 


Fig.  6. — Mean  Monthly  Atmospheric  Pressure.     (See  Table  VI). 


MARYLAND  WEATHER   SERVICE. 


VOLUME  2,  PLATE  II. 


MARYLAXD    WEATHER    SERVICE 


45 


The  progression  is  marked  by  successive  waves  varying  in  period  from 
two  to  eight  or  ten  days'  duration,  which  persist  even  in  the  average 
dailv  values  for  30  vears.     The  variation  from  dav  to  dav  is  smallest 


TABLE  V.-MEAN  DAILY  BAROMETRIC  PRESSURE, 

Reduced  to  sea  level. 

[In  inches  and  hundredths.] 


29.00  Inches. 


Date. 


9.. 
10.. 
11.. 
12. 
13.. 
14.. 
15.. 
16.. 
17.. 
18 
19.. 
20  . 
21.. 
22. . 

h'.'. 

24.. 
25.. 
26.. 
27. . 
2S'.'. 
29.. 
30.. 
31.. 


Average 


Amplitude. 


Jan.    Feb. 


1.14 


.19 


1.14 
1.21 
1.06 
1.10 
1.22 
1.17 
1.14 
1.10 
1.11 
1.17 
1.13 
1.08 
1.04 
1.14 
1.16 
1.12 
1.12 
1.06 
1.08 
1.08 
1.03 
1.04 
1.08 
1.16 
1.05 
1.08 
1.15 
1.14 
.96 


1.11 


.18 


Mar. :  Apr. 


1.07 
1.03 
1.04 
1.06 
1.16 
1.13 
1.09 
1.11 
1.02 
1.05 
1.08 
1.01 
1.03 
1.10 
1.09 
1.04 
1.08 
1.05 
.96 
.95 
1.00 
1.02 
1.03 
1.09 
1.09 
.99 
.97 
.97 
1.02 
1.04 
1.02 


1.04 


1.03 
1.00 
1.00 
.99 
1.02 
1.04 
1.04 
1.05 
l.OI 
1.00 
1.03 
1.08 
1.03 
.98 
.96 
1.03 
1.07 
1.04 
1.02 
1.03 
1.08 
1.08 
1.03 
1.03 
1.03 
1.03 
1.03 
.96 
.96 
1.00 


1.02 


.12 


May 


1.00 

.98 

1.02 

1.01 

.99 

.99 

1.05 

1.03 

1.01 

1.01 

1.01 

1.00 

.99 

1.01 

.99 

1.00 

1.04 

1.03 

.98 

.99 

1.00 

1.01 

1.03 

1.01 

.97 

.97 

.94 

.96 

1.01 

.99 


1.00 


June 


1.01 

1.03 

1.01 

.99 

.99 

l.dO 

1.00 

1.00 

.98 

1.00 

1.01 

1.01 

1.01 

1.03 

1.03 

1.03 

.97 

.97 

.98 

.99 

.96 

.95 

1.00 

1.00 

.98 

.98 

.99 

.95 

.97 

1.02 


0.99 


.08 


July   Aug.   Sept.   Oct.    Nov.  Dec. 


1.04 

1.03 

1.00 

.99 

.99 

1.03 

1.03 

.98 

.96 

.99 

1.00 

1.00 

.96 

.98 

.96 

.96 

.99 

.99 

1.00 

1.01 

1.01 

1.01 

1.02 

1.03 

1.00 

.97 

.98 

1.00 

.98 

.98 

1.00 


0.99 


1.00 
1.00 

.99 
1.03 
1.04 
1.03 
1.01 
1.01 
1.01 

.99 
1.00 
1.00 
1.00 

.99 
1.02 
1.03 
1.03 
1.00 
1.00 
1.02 

.99 
1.01 
1.03 
1.03 
1.01 
1.05 
1.06 
1.07 
1.04 
1.02 
1.05 


1.03 


.08 


1.09 
1.10 
1.08 
1.08 
1.09 
1.08 
1.08 
1.10 
1.12 
1.13 
1.11 
1.08 
1.07 
1.12 
1.10 
1.04 
1.06 
1.08 
1.02 
1.07 
1.14 
l.!3 
1.09 
1.13 
1.12 
1.07 
1.09 
1.12 
1.09 
1.10 


1.09 


1.11 
1.13 
1.11 

1.04 
1.05 
1.05 
1.09 
1.09 
1.12 
1.11 
1.11 
1.11 
1.08 
1.07 
1.13 
1.11 
1.11 
1.11 
1.11 
1.11 
1.13 
1.13 
1.07 
1.12 
1.14 
1.08 
1.04 
1.07 
1.05 
1.09 
1.06 


1.09 


1.11 
1.10 
1.13 
1.17 
1.15 
1.17 
1.15 
1.08 
1.06 
1.05 
1.09 
1.13 
1.13 
1.12 
1.10 
1.20 
1.18 
1.13 
1.04 
1.09 
1.13 
1.14 
1.06 
1.12 
1.12 
1.11 
1.11 
1.13 
1.12 
1.17 


1.12 


.10 


1.16 
1.15 
1.13 
1.11 
1.08 
1.10 
1.10 
1  14 
1.16 
1.13 
1.10 
1.13 
1.09 
1.10 
1.13 
1.16 
I.IT 
1.15 
1  22 
1.22 
1.17 
1.15 
1.15 
1.16 
1.15 
1.05 
1.06 
1.17 
1.11 
1.17 
1.17 


1.14 


.17 


Average  for  the  year  30.063  inches. 


Average  amplitude 0.05  inches. 


Table  V  shows  the  average  sea-level  barometric  pressure  for  each  day  of  the 
year.  The  period  of  observation  covers  the  30  years  from  1871-1900.  The 
daily  mean  is  based  on  three  readings  of  the  mercurial  barometer  at  about 

7  a.  m.,  3  p.  m.  and  11  p.  m.,  from  1871  to  June  1888,  and  on  two  readings  at 

8  a.  m.  and  8  p.  ra.,  from  July  1888  to  1900.  The  correction  for  diurnal  var- 
iation has  not  been  applied,  but  this  is  extremely  small  for  the  observations 
made  at  7  a.  m.,  3  p.  m.,  and  11  p.  m.,  and  about  -f  .01  inch  for  the  series  of 
readings  at  8  a.  m.  and  8  p.  m.  "The  number  29.00  should  be  added  to  each 
of  the  figures  in  the  body  of  the  table.  The  monthly  range  of  the  mean  daily 
pressure  is  indicated  in  the  last  line  of  the  table.  The  figures  of  this  table 
are  also  represented  graphically  in  curve  D  of  Plate  3. 


46 


THE    CLIMATE    OF    BALTIMORE 


M  1875 


Fig.  7.  — "Variations  in  the  Mean  Monthly  Pressure  (Expressed  as  Departures  from 
the  Normal  Values  for  the  Month,  ia  Thousandths  of  an  Inch  of  Mercury).  See 
Table  YIL 


MARYLAND    WEATHER    SERVICE  47 

during  the  summer  months  when  it  is  generally  less  than  0.05  inch, 
and  greatest  in  the  winter  months,  when  it  rises  to  0.10  inch,  and,  occa- 
sionally, to  0.15  inch.  To  what  extent  these  irregular  interdiurnal 
variations  would  be  eliminated  in  a  longer  series  of  observations  is  a 
matter  of  conjecture.  To  a  marked  extent  at  least  they  are  probably 
persistent  and  due  to  a  periodic  recurrence  of  certain  types  of  weather 
at  certain  seasons  of  the  year. 

Of  special  interest  is  the  comparatively  rapid  rise  in  pressure  from 
August  to  September,  and  the  arrested  upward  movement  in  October, 
more  clearly  shown  in  Fig.  6,  constructed  from  monthly  averages,  than 
in  the  serrated  curve  of  daily  means. 

The  barometric  waves  of  short  period  vary  greatly  in  length  and  are 
not  generally  sharply  defined,  but  in  most  instances  they  extend  over 
a  period  of  three  and  a  half  to  four  days,  and  are  accompanied  by  in- 
verse variations  of  temperature  as  is  clearly  shown  in  the  temperature 
and  pressure  curves  of  Plate  III.  The  individual  features  of  these 
waves  are  shown  in  Plate  II,  in  which  actual  tracings  of  the  baro- 
graph are  reproduced  as  representative  types  for  the  different  seasons 
of  the  3^ear.  The  great  variability  of  barometric  conditions  in  the 
winter  months  and  the  comparatively  uniform  conditions  in  the  sum- 
mer months  are  here  shown  in  strong  contrast.  The  curve  representing 
the  conditions  for  the  week  ending  August  13th,  1900,  is  almost  entirely 
free  from  irregular  or  non-periodic  fluctuations,  permitting  the  diurnal 
variations  to  be  plainly  recognized. 

AVERAGE    MONTHLY    AND    ANNUAL    PRESSURE. 

In  an  elaborate  report  on  barometry,'  Professor  Bigelow  has  discussed 
in  detail  the  reduction  of  barometric  observations  at  Weather  Bureau 
stations  in  the  United  States.  In  this  report  all  observations  from  1873 
to  1899  have  been  reduced  to  the  epoch  of  January  1,  1900.  During 
this  long  period  several  different  methods  of  reduction  had  been  em- 
ployed, resulting  in  series  of  observations  not  strictly  comparable.  In 
order  to  obtain  comparable  values  all  reduction^  were  recomputed  and 

'  Bigelow,  F.  H.  The  Reduction  of  Barometric  Pressure  Observations  at 
Stations  of  the  United  States  Weather  Bureau.  Vol.  II  of  the  Report  of  the 
Chief  of  the  Weather  Bureau  for  1900. 


48 


THE    CLIMATE    OF    BALTIMORE 


TABLE  VI.-MEAN  MONTHLY  STATION  PRESSURE  REDUCED  TO  THE  WEATHER 
BUREAU  SrSTEM  FOR  THE  EPOCH  JANUARY  1,  1900. 

[Inches  and  thousandths.] 

Lat.  39°  18'  N.,  Long.  76°  3~'=5  hrs.,  6  m.  W.  of  Gr.    Elevation  above  mean  sea  level  123.3 

feet.     Gravity  corr.  —  .01.5. 
29.000  inches. 


A^ear.  j  Jan.    Feb.    Mar.   Apr. 


May  June 


1873 !  0.971  0.886!  0.8841  0.808'  0.866  0.859 

1874 !  1.053  1.0:il   .879   .8951  .796 

1875 1.083   .986   .947   .833   .835;  .874 


1876. 
1877. 
1878. 
1879. 
1880. 

1881. 

1882. 
1883. 
1884. 
1885. 

1886. 

1887. 

1888 

1889. 

1890. 

1891. 
1892  . 
1893. 
1894. 
1895. 

1896. 
1897. 
1898. 
1899. 
1900. 

1901  . 

1902  . 
1903. 


Mean  (1873-99). 


Corr.  for  sea-  { 
level ) 


1.044 

1.042 

.963 

.963 

LOGO 

1.023 
1.048 
1.075 
1.025 
1.006 

.928 
.934 

1.070 
.912 

1.080 

.921 
.917 
.847 
1.023 
.909 

1.034 
1.009 

.895 
1.032 

.951 


.995 


.127 


1.014 
.972 
.831 
.974 
.993 

1.063 
1.025 
1.155 

.958 


.957 
1.080 

.958 
1.026 

.956 

.954 
.993 
.996 


.764 
.935 
.969 

.918 
.896 


.762 
.947 


.968 


.137 


.918 
.974 
.841 
.980 
.943 

.642 
.983 
.873 
.909 
.922 

.819 
.852 
.942 

.783 
.908 

.949 

.876 
.920 
.963 

.874 

.864] 

.938 

1.073 

.8161 


.855 


.851 


.809 
.891 


.907 
.889 
.756 
.906 


.870 

.983 
.819 

.965 

.874 
.943 
.891 

.895 


.9811 
.960 

.828, 
.939 

.884 

.8011 
.803 
.761 


.938      .868 
.884!     .865 


.796: 

.939 
.929 

.907 
.879 
.839 
.824 
.814 

.78l! 
.884 
.843 
.816 
.819 

.9011 
.838 
.759 
.793 
.933 
I 
.877 
.831 
.793 
.913 
.840 

.726 
.894 
.994 


.853 


.773 
.783 
.841 
.935 

.877 

.839 
.864 
.801 


.920 

.839 
.823 
.863 
.903 
.831 

.858 

.783 
.792 


.899,     .877     .852      .851 


.136      .133      .133      .139 


Mean,  sea-level    1.122    1.105    1.025    1.010      .985      .990 


July  Aug.  Sept.'  Oct.    Nov. 


Dec. 


0.888  0.916'  0.959!  0.934'  0.865  1.039 
.871  .884  .940'  .981  1.056  1.U45 
.838      .873,     .934'     .900      .993      .925 


.877i 
.830 
.811 
.848 
.853 

I 
.817 
.894 
.856 
.744 
.833 

I 
.800 
.830 
.879 
.838 
.880 


.934 
.834 
.870 
.840 


.814 
.899 
.839 
.856 

.818 
.871' 


.8341 
.761 
.831 

.918: 

.894! 
.9001 
.905} 
.9161 

.833, 

.8411 
.847! 
.859 1 
.930, 
.878 

.858 

.865 1 
.834 1 

.881 
.838; 

.901 
.843 

.873 
.831 

.895 


.850 
.953! 

.9771 

1.013 

.918, 


.906 
.935 


.857 


1.043 

.984' 


l.OlO 
1.1171 


.928 
1.077 

.911 
1.0.50 

.928 


.941'  1.013   1.065!  1.037 

.938,  .964'  1.053;  1.015 

.9.34  1.036   1.0.53  1.004 

.968  1.016      .964|  1.047 

.936  .8711     .830  .911 


.984!  1.028! 

.976  .9101 

.918  .848 

.881  .886: 

.971!  .771! 


.898  1.004 

.953,  .996 

.992!  .941 

.932  1.022 

.936:  .928 


1.015' 

1.0241 

.910 

.949 

.9211 

.895! 
1.006 
.9421 
.9211 
.941! 


.955,  1.036 

.895'  .938 

.959!  .980' 

.862'  .976! 

.914  1.033; 


.910 
.990 


1.053 
1.029, 


1.063' 
.974 
.953 
.926 
.918 


.883  .931  1.033  .918 
.833  .904  .9411  .951 
.855   1.009      .9171     .964 


1.054 

.940 

1.004 

1.000 

.983 

1.053 
.947 
.928 
.962 
.966 

.950 
.971 


Year 


.850      .869      .945     .942      .976,     .988  29.917 

1  :  1  I 


.130      .136      .129      .138      .134      .13" 


.980'  1.005    1.075'  1.080,  1.110    1.125  30.050 


.133 


Table  VI  presents  the  average  monthly  and  annual  station  pressures  for 
each  month  and  year  from  1873  to  1899,  as  recorded  in  Professor  Bigelow's 
Report  on  Barometry,  wjth  the  addition  of  values  for  1900  to  1903.  All  ob- 
servations used  in  this  table  were  reduced  to  the  same  plane  (123  feet  above 
mean  tide),  to  the  true  mean  of  24  hourly  observations 'and  corrected  for  the 
force  of  gravity  at  the  Station.  The  values  in  the  footings  of  the  table 
are  Professor  Bigelow's  "  normals  "  for  the  period  1873  to  1899. 


MARYLAND    WEATHER    SERVICE 


49 


uniform  corrections  for  gravity  and  for  diurnal  variation  were  applied 
by  Professor  Bigelow.  The  corrected  monthly  and  annual  means  for 
Baltimore  as  given  by  Professor  Bigelow  are  reproduced  in  Table  VI 
on  page  -iS,  with  the  additional  values  for  the  years  1900  to  1903,  simi- 
larly reduced.     For  the  methods  employed  in  the  reduction  of  observa- 

TABLE  VII.-DEPARTURES  FROM  AVERAGE  STATION  PRESSURE. 
[In  thousandths  of  an  inch.] 


Jan. 


Feb.   Mar. 


Apr. 


May 


1873 !— .034!— .082— .015— .069 +.014 

1874 ! +  .058 +.053 -.030+. 018 -.05B 

1875 |  +  .088|  +  .018|  +  .048-.044-.017 

1876 '  +  .049  +.046  +.019  +.011  +.076 

1877 +.047 +.004 +.075 -.036 +.032 

1878 -.033  —.137  -.058  —.188  —.0,56 

1879 -.032  +.006  +.081  -.068  +.087 

1880 I +  .065 +.025 +.044, +  .014  +.077 

1881 '  +  .028+.095— .2.571— .091  +.055 

1882 +.053  +.057  +.084  +.030  +.027 

1883 I  +  .080+.187— .026+.012— .013 

1884 I  +  .030— .010  +  .010|— .121  —.028 

1885 +  .0U|— .093:  +  .023  +  .029  —.038 


1886. 
1887. 


1889.. 
1890.. 


1891. 
1892. 
1893. 
1894. 
1895. 

1896. 
1897. 
1898. 
1899. 
1900. 

1901. 
1902. 
1903. 


.— .067 -.011 -.080 +.072— .071 
.  I— .071  +.112— .047— .007  +.032 
.  +.0751— .010 +.043  +.106— .010 
.  — .083'  +  . 058— .116]— .0.58— .03« 
.  +.085— .0131 +  .009 +.088— .033 


-.074 -.014  +.050 
-.078|  +  .024-.023 
-.148 +.028, +  .021 
+  .028 +.018 +.064 
.^.086,— .089 -.026 


-.003' +  .049 
+  .065— .014 
+  .014 -.093 
+  .018— .060 
—.008+. 071 


June 


+  .008 
—.023 
+  .023 

+  .017 
+  .014 

—  .063 
+  .002 
+  .014 

-.078 

—  .068 

—  .010 
+  .084 
+  .026 

—.022 
+  .013 
-.050 
+  .031 
-.006 

-.024 
-.004 
+  .015 
+  .011 
+  .069 


+  .038 
+  .021 
-.012 


+  .027  +.067  — .0P6[— .036 

—  .020— .045  +.006|  — .007 

.039! -.108  +.031  —.040 

.002— .038  +.066!+. 101 

+  .002 +.049 —.028, +  .042 


Means. 


Dept. 


'-.0391— .204 —.035 +.104  +.025— .013 
+  .014  — .0:33  +.039  J-.OKi  -.031  -.028 
—  .100  +.001  +.174  -.049  — .059  +.01S 
+  .0;j7i-.0.50— .OKS  +.062  +.0<;0l  +  .O51 
-.044 -.072 —.011  +.0371 -.012 -.020 


.107 
.020 
.129 


-.140'-. 113 

—  .206— .044 

—  .021  +.169 


.078 


.076— .126 
.074' +  .042 
.116 +.143 


.852 


+  .00' 
—.069 
-.059 


.851 


051  —.018  —.040  -.065  -.066  -.067 


July 


Aug.  Sept 


+  .047 
+  .015 
+  .004 


Oct. 


+  .013 -.008 
-.006;  +  . 039 
-.023 -.042 


.033' +  .025 
+  .044 +.031 

+  .006 +.036 

—  .106; +  .047 
-.027-. 037 

-.050— .028 

—  .020— .022 
+  .0291  — .010 
-.022  +.061 
+  .030  +.009 


+  .038 
+  .074 
-.016 
+  .020 


-.011 
-.004 
—  .035 
+  .012 


-.010,-. 041 

+  .039 +.032 
-.036! -.026 
+  .049  +.004 
-.011  —.038 
+  .006  +.026 


-.032 
+  .021 
-.032 


.850 


+  .014 
-.03' 
-.014 


-.005  +.071 
-.018  +.022 
-.012!  +  . 094 
+  .022: +  .074 
-.010-. 071 

+  .a38  +.086 
+  .030— .03; 
-.028!  — .094 
—  .065!— .056 
+  .025 -.171 


Nov.   Dec. 


-.111  +.051 

+  .080 +.057 
+  .017|— .063 

-.119!— .060 
+  .020! +  .089 
-.109— .077 
+  .064  +.062 
+  .141J-.060 

+  .089! +  .049 
+  .076 +.027 
+  .076  +.016 

—  .012, +  .059 
-.146-.07 

—.078'  +  . 016 

—  .023! +  .008 
+  .016— .047 
-.044 +.034 
—.040— .060 


+  .069'  +  .013+.C60!  +  .066 
+  .078— .047 -.038— .048 
-.036  +.017  +.004  +.016 
+  .ro3-.080  .000+. 012 
-.025!— .038+.057|-.005 

-.0511-. 033 +.087, +  .065 
+  .060 +.048!-. 003 -.041 
-.004  +.030  —.023  —.060 
-.025  +.111  -.050 —.026 
-.005  +.087— .058 -.033 


-.015 
-.043 
+  .063 


+  .091 
-.001 
-.025 


946      .943 


.048     .029 


.G25 


-.058-.038 
-.025  -.017 
—.012  —.061 


.059 


.988 


.071 


Year 


-.011 

.020 

+  .001 

+  .001 

+  .008 

.073 

+  .028 
+  .033 

.004 
+  .031 
+  .038 

.005 
-.033 

015 
-.001 
+  .002 

024 
-.006 

+  .019 
-.001 
-.017 
+  .005 
-.009 

+  .005 
+  .005 
-.001 
+  .004 
—.009 

-.048 
—  .038 
-.007 


19.917 


Table  VII  presents  the  average  monthly  and  annual  pressures  expressed  in 
terms  of  departures,  in  thousandths  of  an  inch,  from  the  normal  values  for 
the  period  1873-1899.  The  normal  for  each  month  and  for  the  year  is  shown 
in  the  first  line  of  lootings,  and  the  departures  of  the  monthly  normals  from 
the  annual  normal  are  shown  in  the  last  line.  These  departures  are  also 
graphically  shown  in  Fig.  7  and  Fig.  8. 


50 


THE    CLIMATE    OF    BALTIMORE 


tions  and  for  further  particulars  in  reference  to  the  Baltimore  pres- 
sure data  the  report  of  Professor  Bigelow  should  be  consulted,  espe- 
cially pages  176,  646  and  798. 

In  Table  VII  the  mean  monthly  and  annual  pressures  from  1873  to 
1903  are  given  in  terms  of  departures  from  the  monthly  and  annual 
normal  values.  The  normal  monthly  and  annual  pressures  derived 
from  the  mean  of  the  daily  averages  (see  Table  V)  differ  somewhat 
from  those  derived  from  Professor  Bigelow's  monthly  means  (see  Table 
VI)  after  reducing  the  latter  to  sea-level.  This  discrepancy  is  due  to 
the  fact  that  the  daily  means  were  taken  directly  from  the  original 
record  of  observations  of  the  Baltimore  Office  of  the  Weather  Bureau 
to  which  the  correction  for  diurnal  variation  had  not  been  applied. 


IBTr  1875 


Fig.  8. — Annual  Variations  of  Pressure  Expressed  as  Departures  from  the  Normal 
Value.     (See  Table  VII.) 


ANNUAL   AND   SECULAR   VARIATIONS    OF    PRESSURE. 

The  average  atmospheric  pressure  of  a  year  is  by  no  means  a  constant 
quantity.  The  fluctuations  in  value  from  year  to  year  are  sometimes 
considerable.  This  is  most  readily  recognized  when  the  variations  are 
graphically  presented  as  in  Fig.  8  on  page  50.  Here  the  Baltimore  ob- 
servations are  plotted  in  terms  of  annual  departures  from  the  normal 
value  for  the  entire  period  from  1871  to  1903.  The  amplitude  of 
fluctuation  is  expressed  in  thousandths  of  an  inch.  The  resulting  curve 
presents  a  series  of  waves  or  surges  varying  in  amplitude  from  a  few 
thousandths  of  an  inch  to  nearly  one-tenth  of  an  inch.  The  period  of 
oscillation  also  varies  considerably,  yet  there  is  a  remarkable  uniformity 
in  the  length  of  these  periods.  Measuring  from  crest  to  crest  and  from 
hollow  to  hollow  of  these  waves  Ave  have  the  following  figures  repre- 
senting the  periods  in  years  and  fractions: 


MAKYLAXD    "WEATHER    SERVICE 

51 

Number  of  Years. 

Mean. 

From  crest  to  crest 

(from  1871). 

4 

4 

4 

4 

5.5 

4.5 

4 

3.5 

3.5 

5.5 

4.2 

From  hollow  to  hollow 

(from  1873). 

3.5 

3.5 

4 

4 

5 

5 

5 

3 

4 

4 

4.1 

Since  1871,  the  beginning  of  the  series  of  observations  at  Baltimore, 
no  crest  of  a  wave  has  been  heloiv  the  normal  value  for  the  entire  period 
and  no  hollow  has  been  al)ove  the  normal  level.  In  order  to  fall  into 
harmony  with  this  series  the  year  1903  should  form  the  crest  of  a 
wave  and  be  followed  by  approximately  equal  pressure  in  1904  and 
lower  pressure  in  1905;  but  we  must  not  overlook  the  fact  that  it  is  the 
unexpected  which  is  most  likely  to  follow  a  long-range  forecast.  Tlie 
period  from  1871  to  1903  includes  ten  waves  with  an  average  length 
from  crest  to  crest,  or  from  hollow  to  hollow,  of  slightly  over  4  years, 
the  limits  of  variability  being  three  years  and  five  years  and  a  half. 

A  conspicuous  feature  of  the  annual  variation  of  pressure  at  Balti- 
more is  the  abnormally  low  pressure  of  1878.  The  departure  in  this 
year  was  nearly  five  times  the  average  annual  variability.  Upon  first 
examination  it  appears  suspiciously  large.  It  is,  however,  substantiated 
by  similar  departures,  though  not  so  marked,  at  stations  in  all  parts  of 
the  United  States.  A  few  of  the  larger  departures  occurring  in  this 
year  are  here  given : 


DEPARTURES   FROM    NORMAL   PRESSURE   IX    1878. 


Baltimore    — .072 

Washington — .057 

New  York — .052 

Cincinnati — .080 

St.  Louis —.049 


New  Orleans  . 

St.  Paul 

San  Francisco 
Key  West  .  . .  . 
Boston 


—.068 
—.052 
—.058 
—.059 
—.056 


The  abnormally  low  pressure  evidently  extended  over  a  very  large 
territory  in  1878.  At  Baltimore  the  pressure  was  decidedly  below  the 
average  during  every  month  of  the  year,  excepting  September,  when  it 
was  but  slightly  above.  Usually  there  is  considerable  fluctuation  above 
and  below  the  annual  average  during  the  course  of  the  year.  In  1901 
the  average  pressure  was  also  abnormally  low,  but  not  as  low  as  in  1878. 

Another  marked  feature  of  the  curve  is  the  steadv  diminution  in  the 


52 


THE    CLIMATE    OF    BALTIMORE 


amplitude  of  fluctuation  from  18T8  to  1900,  diminishing  with  consider- 
able uniformity  from  nearly  one-tenth  of  an  inch  to  about  one-hundredth 
of  an  inch.     Since  1900  the  amplitude  has  again  increased.     The  curve 


30  000  inches. 


TABLE  VIII.-MAXIMUM  STATION  PRESSUKES. 
[In  inches  and  hundredths.] 


Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Year 

1875 

.68 
.52 
.51 
.61 
.44 

.61 

.57 
.85 
.50 
.26 
.70 

.58 

.37 
.43 
.48 
.40 

.65 

.40 

.32 
.43 
.26 

.20 
.37 

.32 

.24 
.32 
.30 
.14 
.42 

.31 

.13 

.12 
.19 
.11 
.22 

.22 

.16 
.18 
.16 
.06 
.23 

.12 

.20 
.16 
.16 
.10 
.13 

.34 

.35 
.13 
.24 
.40 
.36 

.31 

.48 
.36 
.41 
.29 
.75 

.41 

.61 
.30 
.51 
.46 
.50 

.68 

•61 
.59 
.51 
.49 
.56 

.47 

.68 

1876 

.85 

1877 

.51 

1878 

.61 

1879 

1880 

.68 

1881 

.61 

.73 

.29 

.30 

.41 

.12 

.16 

.22 

.24 

.52 

.66 

.57 

.73 

1882 

.83 

.68 

.65 

.43 

..38 

.18 

.23 

.26 

2'i 

.29 

.51 

.48 

.83 

1883 

..58 

.73 

.48 

.37 

.28 

.40 

.15 

.20 

.38 

.56 

.60 

.57 

.73 

1884 

.79 

.68 

.40 

.16 

.24 

.41 

.00 

.24 

.41 

.53 

.38 

.59 

.79 

1885 

.75 

.41 

..38 

..50 

.10 

.21 

.09 

.15 

.27 

.24 

.29 

.70 

.75 

1886 

.77 

.49 

.36 

.43 

.21 

.18 

.13 

.25 

.34 

.41 

.38 

.46 

.77 

1887 

.57 

.81 

.65 

.55 

.23 

.26 

.11 

.19 

.35 

.39 

.74 

.83 

.83 

1888 

.73 

.57 

.44 

.51 

.19 

.16 

.15 

.19 

.44 

.31 

.63 

.53 

.73 

1889 

.51 

.78 

.45 

.39 

.28 

.35 

.16 

.18 

.23 

.28 

.59 

.73 

.78 

1890 

.62 

.47 

.49 

.54 

.21 

.28 

.19 

.15 

.27 

.24 

.28 

.51 

.    .62 

1891 

.40 

.56 

.55 

.34 

.26 

.10 

.18 

.08 

.27 

.38 

.67 

.51 

.67 

1892 

.49 

.57 

.37 

.35 

.17 

.11 

.36 

.04 

.27 

.31 

.31 

.46 

.01 

1893 

.27 

.64 

.46 

.31 

.23 

.16 

.10 

.07 

.19 

.37 

..53 

.71 

.71 

1894 

.48 
.34 

.66 
.40 

.45 

.,38 

.28 

.48 

.30 
.17 

.15 
.32 

.06 
.10 

.05 
.12 

.38 
.23 

.23 
.40 

.59 
.51 

.60 

.66 

1895 

.60 

1896 

.38 

.39 

.49 

.38 

.24 

.12 

.22 

.20 

.22 

.36 

.62 

.78 

.78 

1897 

..59 

.51 

.58 

.50 

.28 

.13 

.17 

.07 

.27 

.48 

.46 

.42 

.59 

1898 

.40 

.50 

.52 

.12 

.16 

.15 

.20 

.12 

.29 

.33 

.42 

.46 

.52 

1899 

.84 

.56 

.30 

.27 

.20 

.11 

.12 

.11 

.29 

.41 

.40 

.49 

.84 

1900 

.44 

.58 

.47 

.30 

.21 

.04 

.08 

.18 

.20 

.30 

.52 

.36 

.58 

1901 

.61 

.23 

.22 

.33 

.03 

.08 

.04 

.06 

.33 

.47 

.34 

.38 

.61 

1902 

.79 

.24 

.32 

.18 

.26 

.35 

.11 

.11 

.32 

.43 

.38 

.56 

.79 

1903 

.42 

.49 

.43 

.31 

.34 

.18 

.35 

.27 

.30 

.21 

.57 

.46 

.57 

Extremes 

.84 

.85 

.65 

.55 

.42 

.41 

.36 

.34 

.44 

.75 

.74 

.83 

.85 

Table  VIII  presents  the  highest  station  pressure  observed  at  any  of  the 
regular  hours  of  observation  for  each  month  and  for  the  entire  year,  from 
1875  to  1903,  together  with  the  absolute  extremes  for  each  month  and  for  the 
entire  period  of  observation.  The  absolute  extremes  are  also  indicated  in 
Fig.  9  curve  '(a).  The  number  30.00  should  be  added  to  each  of  the  figures 
in  the  table. 


also  shows  some  suggestions  of  a  wave  of  greater  period.  From  1885  to 

1899  there  seems  to  have  been  a  gradual  rise  in  pressure.  The  Baltimore 

series   of   observations   is,  however,  too  short  to   place  much   reliance 
upon  the  evidence  of  a  long  period  of  variation. 


IMARYLAXD    "WEATHER    SERVICE 


53 


THE  AVERAGE  VARIABILITY  OF  PRESSURE. 

Some  interesting  facts  regarding  the  variability  of  pressure  conditions 
are  revealed  in  Talkie  Til  showing  the  departures  of  monthly  average 
pressures  from  18T3  to  1903.     The  month  of  greatest  variability  is  March 

TABLE  IX.-MIXIMUM  STATION  PRESSURES. 

[In  inches  and  hundredths.] 
29.000  inches. 


Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

1 
Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Tear 

1875 

..55 

.36 

.36 

.52 

.35 

.66 

.61 

.64 

.59 

.49 

.52 

.37 

.35 

1876 

.49 
.31 
.39 
.29 

.48 

.20 
.38 
.28 
.43 

.27 

.21 
.02 
.18 
.34 

.26 

.48 
.36 
.22 

.52 

.58 
.46 
.55 
.64 

.73 

.64 
.52  1 
.40 
.44 

.59 

.72 
.64 
.55 
.51 

.59 

.72 
.63 
.56 
.59 

.66 

.18 
.66 
.49 
.65 

.59 

.49 

.29 

8.74 

.54 

.42 

.48 

.38 

8.90 

.45 

.41 

.01 

.29 

8.65 

.57 

..59 

.01 

1877 

.02 

1878 

28.65 

1879 

.29 

1880 

.26 

1881 

.26 

.32 

.no 

.46 

.56 

.54 

.60 

.60 

.77 

.50 

.64 

.34 

.00 

1882 

.29 

.26 

.48 

.31 

.46 

.47 

.57 

.56 

.50 

.74 

.69 

.60 

.26 

1883 

.54 

.65 

.29 

.56 

.29 

.60 

.64 

.61 

.50 

.44 

.69 

.48 

.29 

1884 

.17 

.25 

.45 

•1*. 

.57 

.63 

.55 

.64 

.66 

.66 

.42 

.45 

.14 

1885 

.40 

.19 

..53 

.46 

.44 

.43  i 

.61 

..52 

.42 

.02 

..53 

.25 

.02 

1886 

8.91 

.24 

.20 

.29 

.47 

.47 

.63 

.59 

.&i 

.73 

.41 

.44 

28.91 

1887 

.37 

.30 

.35 

.22 

.52 

.55 

.62 

.64 

.54 

.48 

.39 

.32 

•to 

1888 

.62 

.44 

.34 

.48 

.59 

.«3 

.58 

.35 

.60 

.42 

.38 

.85 

.34 

1889 

.13 

.47 

.35 

.22 

.63 

.59 

.60 

.66 

.47 

.44 

.40 

.54 

.13 

1890 

.63 

.54 

.38 

.42 

.54 

.67 

.64 

.64 

.82 

.35 

.60 

.36 

..33 

1891 

.12 

.37 

.37 

.38 

.66 

.53 

.61 

.56 

.76 

.00 

.28 

.48 

.12 

1892 

.14 

.17 

.16 

.52 

.44 

.50 

.63 

.59 

.59 

.47 

.47 

.41 

.14 

1893 

.07 

.12 

.27 

.35 

.26 

.48 

..56 

.39 

.54 

.01 

.60 

.43 

.01 

1894 

.22 

.39 

.50 

.34 

.40 

.52 

.58 

.66 

.55 

.20 

.54 

.23 

.20 

1895 

.17 

.22 

.35 

.22 

.48 

.64 

.56 

.61 

.60 

.52 

.35 

.31 

.17 

1896 

.45 

8.81 

8.99 

.57 

.55 

.43 

.59 

.70 

.54 

.00 

..50 

.61 

28.81 

1897 

.54 

.43 

.25 

.47 

.40 

.57 

..50 

.00 

.70 

..54 

.28 

.36 

.25 

1898 

.29 

.26 

.69 

.33 

.36 

.52 

.61 

.68 

..59 

.52 

.29 

.11 

.11 

1899 

.18 

.22 
.14 
.32 

.39 

.14 
.44 
.14 

.04 

.22 
.21 
.19 

.48 

.54 

8.97 

.12 

.56 

.30 
.42 
.58 

.66 

.57 
.63 
.34 

.68 

.46 
.66 
.64 

.60 

.75 
.72 
..59 

.57 

.55 
.54 
.64 

.35 

.72 
..53 
.43 

.31 

.19 
.25 

.30 

.13 

.30 
.38 
.20 

.03 

1900 

.14 

1901 

28.97 

1902 

.12 

1903 

.21 

.17 

.46 

.21 

.76 

.50 

.49 

.61 

.62 

.48 

.52 

.26 

.17 

Extremes 

8.91 

8.81 

8.99 

8.97 

.26 

.34 

.46 

.35 

.18 

8.74 

8.90 

8.65 

28.65 

Table  IX  presents  the  lowest  station  pressure  observed  at  any  of  the  regular 
hours  of  observation  for  each  month  and  for  the  entire  year,  from  1875  to 
1903,  together  with  the  absolute  extremes  for  each  month  and  for  the  entire 
period  of  observation.  The  absolute  extremes  are  also  indicated  in  Fig.  9 
curve  (e).  The  number  29.00  should  be  added  to  all  fractional  numbers  in 
the  table. 

with  an  extreme  limit  of  0.431  inch  and  an  average  variability  of  0.058 
inch.  The  month  of  most  uniform  pressure  is  June  with  an  extreme 
amplitude  of  0.162  inch  and  an  average  variability  of  0.029  inch.  The 
montli  of  December  sbows  a  remarkable  freedom  from  extreme  fluctua- 


54 


THE    CLIMATE    OF    BALTIMORE 


tions,  being  next  to  June  in  this  respect,  while  at  the  same  time  exhibit- 
ing a  fairly  large  average  variability.  In  the  following  table  the  varia- 
bility of  the  average  monthly  and  annual  pressure  at  Baltimore  is  shown 
by  means  of  the  average  plus  or  minus  departures  from  the  normal 
monthly  and  annual  values,  the  greatest  plus  and  minus  departures  and 
the  extreme  variations  of  the  monthly  and  annual  values. 


J 

VI 

\ 

M 

J 

J 

K 

5 

o 

M 

3 

J 

Inches 

1 

, 

(a) 

s 

/ 

-^ 

^ 

s 

X 

V 

/ 

' 

' 

\ 

^ 

\ 

> 

(b) 

K, 

s 

/ 

" 

s 

s. 

/ 

^ 

-" 

\ 

/ 

^ 

.50 

^ 

/ 

^ 

t 

N 

"• 

^ 

^ 

/ 

/ 

ff 

■>«. 

/ 

^ 

■ 

(<=) 

- 

>. 

.- 

— 

■* 

30.00 

■4 

0^ 

' 

M 

K 

... 

r 

^ 

_ 

—i 

.' 

s 

r* 

N 

/ 

, 

L 

s, 

.50 

/ 

/ 

' 

S. 

^ 

S 

f 

/ 

(dl 

. 

. 

\ 

»« 

^ 

■" 

— 

,>• 

r^ 

\ 

/ 

\ 

/ 

\ 

r 

^ 

J 

V 

\ 

(ej 

/ 

29.00 

f 

\, 

/ 

J 

/ 

k. 

^ 

\ 

' 

\ 

/ 

\ 

/ 

^ 

, 

y 

y 

/j 

f 

.50 

_ 

_ 

_ 

Inches 
31.00 

(^) 
fb) 


30,00 


w 


2900 
Ce) 


.50 


Fig.   9. — Monthly  Means  and  Extremes  of  Pressure.     (See  Tables  VIII,   IX 
and  X.) 

VARIABILITY  OF  PRESSURE  CONDITIONS  AT  BALTIMORE. 

(Expressed  as  departures  from  the  normal  values  in  thousandths  of  an  inch.) 


Jan. 

Feb. 

Mar. 
174 

Apr. 

May 

June 

July 

Aug-. 

Sept. 

Oct. 

Nov. 

Dec. 

Year 

Greatest  plus 
departure  (+). 

88 

187 

106 

142 

84 

74 

67 

78 

Ill 

141 

89 

38 

Greatest  minus 
departure  (— ). 

148 

206 

357 

188 

126 

78 

106 

108 

96 

171 

146 

77 

72 

Extreme  am- 
plitude. 

236 

393 

431 

294 

268 

162 

18 

175 

174 

282 

287 

166 

110 

Average  de- 
parture /'  +  \ 

59 

.5.5 

58 

54 

45 

29 

30 

33 

32 

.55 

59 

46 

15 

MARYLAXD    WEATHER    SERVICE 


55 


The  negative  departures  are  far  more  marked  than  the  positive.  Only 
in  May,  June  and  November  have  the  plus  departures  exceeded  the 
minus.     This  contrast  is  particularly  strong  in  the  figures  representing 

TABLE  X.-SUMMARY  OF  PRESSURE  CONDITIONS. 

iln  inches  and  thousandths.] 


Monthly  Means. 

Mean  Monthly  and  Annual 
Extremes. 

Absolute  Extremes. 

Highest  and  Low-         i     . 

1875 

-1903. 

1875-1903 

Means. 

est  Means  as  De- 1          ^ 

partures  from     !       i  -^ 

1873 

to 

1899 

Normal.              .     .:: 
1873-11)03.            ii,  ,  .3 

Average  of 
Extremes. 

Departures 
from  Nor- 
mal. 

o 
til 

a 

2 

Max. 

& 

Min. 

a  ,     n 

21           Zt 

^             |P3     ^ 

. 

tx 

t>l    1      ^i 

CO 

.    1  ■  - '      s 

"^           "S 

m              ^-» 

d 

1 

r"         1-5 

»       s 
5       c 

2         § 

if            £ 

30.000+ 

38. 000  4- 

29.000+ 

1    1 

^.COn-l-  29.000+ 

1 

1 

Jan 

.995 

-h .  088  1 875  - .  1 48  1 893  .  236  . 0.59 

.572        ..307 

J+.577— .688 

1.265 

.843 

1899^     .908 

1886  1.935 

Feb.  .. 

.968 

.  1 87 188:3  - .  206  Vm.  393  .  055 

.557        .298 

.589 -.670 

1.259 

.849 

1876;     .809 

1896  2.040 

Mar.  .. 

.899 

.174  1S98-.  2.57  1881  .431  .0.58 

.443         .291 

.544 -.608 

1.152 

.6.50 

1887]     .993 

1.S96   1.6.58 

Apr... 

.877 

.1061,S88  — .1><81S78  .294  .054 

.353         .363 

.476 -.514 

.99C 

.550 

1887!     .969 

1901    1.581 

May... 

.852 

.142  I903-.126  1901  .268  .045 

.245         ..50  J 

.393-.a5C 

.743 

.424 

1879  J. 257 

1893  1.167 

Juno  .. 

.851 

.084  1884  -  .078 1881  .  162  .029 

.191         ..=^42 

'     .340  — .309 

.64S 

.414 

1884   1.345 

1902  1.069 

July.. 

.850 

.  074  1 892  — .  1 06  1 884  . 1 80  .  030 

.151    1     .594 

:     .301 -.256 

.557 

.3,59 

1892   1.4.57 

1900     .903 

Aug.  .. 

.869 

.0671876  -.108  1878  .175  .OSJ 

.159         .608 

.290 -.261 

..551 

.342 

1880  1.3,50 

il8S8     .993 

Sept... 

.946 

.  078  1 892  - .  0!'6 1 876  . 1 74  .  032 

.293    1     .581 

.347-. 365 

.712 

.440 

1888  1.181 

1876  1.259 

Oct.... 

.943 

.11 11899 -.1711.'-90.'J82. 0.55 

.381         .435 

'     .439  — .507 

.946 

.753 

1879     .739 

il878  2.013 

Nov.  .. 

.976 

.141 1880  — .1461885  .287  .059 

.498         .420 

.522 -.5.56 

1.078 

.740 

1887     .902 

1878  1.8:« 

Dec.  .. 

.9^8 

.089 1877 -.077 1878 

.166.046 

.540 

.353 

.552 -.636 

1.188 

.830 

1887 

.648 

1878  2.183 

Year . . 

.917 

.038 1883 -.072 1878 

.110  .015 

.691 

.120 

.777-.797 

1.574 

.849 

1876 

.648 

.878  3.301 

Table  X  presents  a  summary  of  pressure  conditions,  derived  mostly  from 
the  preceding  tables.  It  shows  the  mean  monthly  values;  the  highest  and 
lowest  mean  values,  with  year  of  occurrence  and  range;  the  mean  variability 
of  the  monthly  means;  the  mean  monthly  and  annual  extremes,  with  their 
respective  departures  from  the  normal  value,  and  their  ranges;  the 
amount  and  year  of  occurrence  of  the  absolute  extreme  values,  and  the  ab- 
solute range  of  pressure  for  each  month  and  year.  Much  of  the  data  con- 
tained in  this  table  is  also  shewn  graphically  In  Fig.  9. 


the  annual  departures.  The  extreme  amplitudes,  or  the  differences  be- 
tween the  highest  and  lowest  average  monthly  values  are  about  six  times 
larger  than  the  average  plus  or  minus  departures.  This  ratio  is  remark- 
ably constant  throughout  the  year,  excepting  the  months  of  December 
and  Januarv  when  the  ratio  falls  to  4. 


56  THE    CLIMATE    OF    BALTIMORE 

EXTREMES    OF    PRESSURE. 

The  extreme  range  of  the  barometer  at  Baltimore  from  1871  to  1903, 
according  to  the  official  records  of  the  United  States  Weather  Bureau, 
is  2.20  inches.  The  highest  ohserred  reading,  namely  30.85  inches,  oc- 
curred on  February  5,  1876,  and  the  lowest,  28.65  inches,  on  December 
10,  1878.  The  barometer  seldom  falls  below  29.00  inches  in  the  Middle 
Atlantic  states;  since  1875  a  lower  reading  has  been  observed  at  Balti- 
more but  once  in  each  of  the  months  of  January,  February,  March,  April, 
October,  November,  and  December,  and  never  in  the  months  of  May  to 
September.  The  very  low  pressures  occur  only  in  connection  with  a 
severe  cyclonic  storm  of  the  winter  type,  or  in  connection  with  tornadoes. 
In  the  center  of  the  extremely  limited  area  of  a  tornado  the  barometer 
has  fallen  to  27.00  inches  or  less  for  a  fcAV  minutes,  but  Baltimore  has 
fortunately  been  visited  but  two  or  three  times  in  the  past  30  years  or 
more  by  these  fierce  and  destructive  storms,  and  then  only  by  a  compara- 
tively mild  type.  Of  the  seven  occasions  referred  to  above  on  which  the 
barometer  fell  below  29.00,  three  occurred  in  the  year  1878,  a  year 
remarkable  for  low  pressures,  two  in  1896,  one  in  1886,  and  one  in  1901. 

The  abnormally  high  pressures  likewise  occur  in  the  winter  months 
only,  the  most  marked  of  them  in  connection  Avith  the  intenser  types  of 
cold  waves.  The  highest  observed  reading  of  the  barometer  occurred 
during  the  cold  wave  of  February,  1876,  when  the  pressure  rose  to  30.85 
inches.  A  detailed  record  of  the  highest  and  lowest  observed  pressures 
for  each  month  and  for  the  year  is  given  in  Tables  Till  and  IX  on  pages 
52  and  53.  For  a  summary  of  averages  and  extreme  conditions  of 
pressure  reference  may  be  made  to  Table  X.  In  Figure  9  some  of  the 
chief  features  of  this  table  are  graphically  shown. 


TEMPEEATUEE  OF  THE  ATMOSPHEEE. 

Introduction. — There  are  certain  factors  which,  in  the  long  run,  de- 
termine the  average  temperature  of  every  locality.  Of  these  the  latitude, 
the  position  of  the  place  with  reference  to  large  land  and  water  areas,  the 
height  above  sea-level,  the  nature  of  the  soil,  and  other  factors  of  minor 
importance  are  constant  and  tend  to  give  to  a  place  a  fixed  mean  temper- 


M AH Y LAND    WEATHER    SERVICE  57 

ature.  Other  factors,  as  wind  direction,  amount  of  cloudiness,  etc.,  vary 
greatly  from  day  to  day  and  from  season  to  season,  and  tend  to  produce  a 
variable  mean  temperature.  In  some  localities,  within  the  tropics  for 
example,  these  variable  factors  become  fairly  constant,  and  enable  us  to 
determine  the  average  temperature  by  means  of  a  comparatively  short 
period  of  observations.  In  others  the  variable  factors  are  large,  as  in  the 
temperate  zones,  especially  in  the  usual  paths  of  cyclonic  disturbances. 
In  such  regions  a  long  series  of  observations  is  often  necessary  to  de- 
termine the  average  temperature  conditions  to  within  1°  or  less.  In  the 
smaller  islands  of  the  tropics  five  or  six  years  of  carefully  made  temper- 
ature records  will  yield  an  annual  mean  value  with  a  probable  error  not 
greater  than  one-tenth  of  a  degree.  In  the  temperate  regions  an  equally 
accurate  annual  mean  niay  require  observations  covering  a  period  of  50 
to  100  years.  In  the  long  run  the  effect  of  the  variable  climatic  factors  is 
eliminated  and  a  given  locality  secures  a  position  upon  the  normal  tem- 
perature chart,  due  to  its  geographical  and  topographical  position  and  the 
nature  of  the  soil.  Baltimore  occupies  a  middle  position  on  the  climatic 
chart  with  average  annual  and  summer  temperatures  3°  or  4°  below  tlie 
average  for  the  entire  globe,  and  a  winter  temperature  about  10°  below. 
The  city  lies  between  a  region  of  equable  temperatures,  the  ocean,  and  one 
of  great  variability,  the  nortli  continental  area.  The  factor  to  which 
is  due  most  of  the  changeable  character  of  the  weather  of  Baltimore, 
causing  a  variability  greater  than  is  its  due  on  account  of  latitude,  is  its 
proximity  to  the  great  transcontinental  storm  paths.  Baltimore  is  within 
the  influence  of  the  barometric  depressions  which  continually  pass  from 
the  northwest,  across  the  Lake  region  and  the  New  England  states,  and 
which  are  accompanied  by  rapid  changes  in  wind  direction  from  the  warm 
southerly  to  the  colder  west  and  northwest  winds. 

Average  Temperatures. 

For  purposes  of  comparison  it  is  essential  to  have  a  standard  of  refer- 
ence. In  discussing  temperature  conditions  the  standard  of  value  is 
usually  assumed  to  be  the  average  daily  temperature.  This  daily  average 
is  derived  from  observations  made  hourly  throughout  the  day  and  niglit. 
Approximate  averages  are  obtained  from  two  or  more  observations  made  at 


58  THE    CLIMATE    OF    BALTIMORE 

such  hours  of  the  day  as  to  give  a  value  more  or  less  closely  agi'eeing  with 
that  derived  from  hourly  observations.  Experience  has  shown  that  fairly 
accurate  daily  averages  may  be  obtained  by  noting  the  temperature  at 
7  a.  m.,  3  p.  m.,  and  9  p.  m.,  or  7  a.  m.,  3  p.  m.,  and  11  p.  m.,  or  10  a.  m., 
and  10  p.  m.,  or  8  a.  m.,  and  8  p.  m.,  or  from  the  highest  and  lowest  tem- 
peratures recorded  during  the  day.  In  later  years  automatically  record- 
ing instruments  have  largely  displaced  direct  observations  permitting  us 
to  obtain  a  daily  mean  temperature  to  any  desired  degree  of  -accuracy  with 
comparatively  little  personal  attention.  Monthly,  seasonal,  and  annual 
means  are  in  turn  derived  from  the  daily  means. 

In  the  discussion  of  temperatures  in  succeeding  pages  we  must  not  lose 
sight  of  the  nature  of  average  values.  They  are  not  real  values  in  the 
sense  of  occurring  in  nature.  When  we  say  that  the  average  temperature 
on  the  4th  of  July  in  Baltimore  is  79°,  we  mean  that  by  adding  together 
the  hourly  temperatures  on  the  4th  of  July  for  a  great  many  years  and 
dividing  by  the  total  number  of  hours  we  obtain  the  value  79°.  This 
may  never  have  been  the  real  average  value  for  the  day  on  any  4th  of  July. 
It  is  simply  an  arithmetical  mean ;  the  real  temperatures  of  the  day  may 
have  had  any  value  from  60°  to  100°  or  more. 

Average  values  are  sometimes  very  misleading  if  sole  reliance  be  placed 
upon  them  to  characterize  the  temperature  conditions  of  a  day  or  a 
season.  Two  seasons  may  have  the  same  mean  temperature  and  yet  be 
totally  different  in  character.  The  summer  of  1898  left  the  impression  of 
an  unusually  warm  season.  The  official  records  show  a  temperature  very 
near  the  average  of  a  period  of  thirty  years  (76°).  The  average  may  be 
obtained  from  any  one  of  a  large  series  of  combinations,  and  our  general 
impression  of  the  character  of  the  season  will  be  determined  by  the 
particular  combination  of  weather  experienced.  The  temperature  may 
lemain  uniformly  near  the  average  throughout  the  season;  there  may  be 
excessively  high  temperatures  of  short  duration  combined  with  longer 
periods  of  moderately  low  temperatures;  or  excessively  low  temperatures 
combined  with  longer  periods  of  moderately  high;  or  there  may  be  very 
high  combined  with  very  low  temperatures,  etc.  All  of  these  combina- 
tions may  produce  a  "  normal "  average,  but  the  personal  effect  will  be 
different  in  each  instance,  and  give  rise  to  a  variety  of  opinions  as  to  the 


MARYLAXD    WEATHER    SERVICE  59 

character  of  the  season.  Disregard  of  such  considerations  frequently 
leads  to  unfavorable  criticism  of  official  records.  Hence  the  figure  rep- 
resenting the  average  temperature  of  a  period  is  not  of  itself  a  safe  cri- 
terion of  the  temperature  conditions ;  the  variability  of  temperature  is  an 
essential  factor  in  revealing  the  character  of  the  period. 

The  XoRiiAL  Hourly  Temperature. 

The  most  familiar,  and  at  the  same  time  most  regular,  feature  of 
changes  in  the  weather  is  the  rise  and  fall  of  temperature  between  sunrise 
and  sunset.  Like  the  pressure  change  it  is  most  regular  in  the  tropical 
regions,  and  diminishes  in  amplitude  with  distance  from  the  equator  until 
it  disappears  by  merging  into  the  annual  change  within  the  Arctic  Circle. 
As  the  amplitude  of  variation  depends  very  largely  upon  the  character  of 
the  surface  upon  whch  the  rays  of  the  sun  fall,  there  are  marked  de- 
partures from  the  general  law  of  decrease  in  amplitude  with  increased 
latitude.  Over  a  water  surface  the  daily  changes  are  small:  over  the 
interior  of  the  continental  areas,  especially  over  sandy  soils  and  in  a 
dry  atmosphere,  they  are  enormously  increased.  The  difference  between 
the  highest  and  lowest  temperature  recorded  during  an  average  day  a 
few  feet  above  the  surface  of  mid-ocean  is  not  ordinarily  more  than  1°  or 
2°,  owing  to  the  relatively  large  absorbing  power  of  water,  and  to  the 
large  quantity  of  heat  employed  in  the  conversion  of  water  into  vapor — 
the  latent  heat  of  evaporation.  The  surface  of  the  soil,  especially  when 
unprotected  by  vegetation,  is  rapidly  warmed  by  the  sun's  rays  and  attains 
a  high  temperature,  owing  to  its  comparatively  low  specific  heat.  The  at- 
mosphere above  such  surfaces  is  in  turn  heated  by  contact  and  bv  con- 
vection currents.  In  consequence  the  difference  between  midday  and 
night  temperatures  over  land  surfaces  is  many  times  larger  than  over 
water  surfaces.  For  any  given  locality  the  diurnal  variation  also  varies 
with  the  season  of  the  year,  following  the  changes  in  the  altitude  of  the 
sun,  and  hence  is  greatest  in  the  summer  months  and  least  in  the  winter 
months. 

The   Baltimore   hourly   observations   of   temperature   extend    over    a 

period  of  ten  years,  affording  ample  data  for  determining  all  phases  of  the 

<liurnal  variation.     'J'he  tracings  of  the  Eichard  thermogra))li  were  cor- 
5 


60 


THE    CLIMATE    OF    BALTIMORE 


rected  at  four  points  each  day  by  means  of  direct  observation  of  a  mer- 
curial thermometer  at  8  a.  m.,  and  8  p.  m.,  and  by  readings  of  the  maxi- 
mum and  minimum  points  reached  by  a  mercurial  maximum  and  an 
alcohol  minimum  thermometer.     The  average  values  for  the  ten  years 


I      234567      8910    II  "ooi.  I      2     3     4    S     6     7     6     8    10   II    trt 


1 

x 

M 

1 

^ 

_ 

p 

-1 

— 1 

*/ 

y 

\ 

/ 

\ 

/ 

\ 

/ 

> 

L 

80 

/ 

\ 

\ 

k«- 

^ 

\ 

V 

. 

1 

s 

■ 

/ 

s 

Ti 

i 

N 

/ 

\ 

'•>< 

f 

N"2 

^ 

y 

■" 

s. 

^ 

m 

t 

t; 

64 

/ 

N 

/ 

\ 

J 

^ 

_, 

\ 

t 

f 

^ 

'S 

v 

60 

/ 

/ 

^ 

''" 

•> 

s> 

N 

k 

r 

/ 

/ 

\ 

V  \ 

d 

1 

/ 

f 

/ 

\\s 

Nfe 

f 

/ 

J 

0 

Sir- 

S 

S 

- 

/ 

J 

^     / 

f 

\ 

s 

55 

;  i 

/ 

/ 

/ 

Va 

S 

k 

^.^ 

y 

> 

y 

^ 

^ 

^ 

^ 

z^ 

/ 

/ 

/ 

s 

S 

^ 

V 

/ 

V 

V 

V 

^ 

^ 

/ 

/ 

S 

iO 

-.    1^ 

^-. 

l/ 

\3^i 

/ 

J^^ 

f 

<>» 

/ 

33 

1 

.^ 

/ 

N 

s. 

/ 

^ 

3S 

/ 

N 

^ 

y 

s 

sr 

- 

, 

/ 

V, 

^ 

X 

'<5 

fe-^ 

/ 

-O^/^ 

/ 

30 

1    P- 

•ar 

*f 

_| 

Fig.   10. — Mean  Hourly  Temperature.     (See  Table  XI.) 


for  each  hour  of  the  day,  for  each  month,  and  for  the  year  are  given  in 
Table  XI,  and  in  Fig.  10  and  Fig.  11. 

The  details  of  changes  in  temperature  from  hour  to  hour  are  best 
shown  in  tabular  form  from  which  the  exact  value  for  each  hour  may  be 
readily  taken.    The  graphic  form,  however,  presents  advantages  in  afford- 


TABLE  XI. 

-MEAN  HOURLY  TEMPEBATUEE. 

Hours. 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov.  Dec. 

1  A.  M 

0 

31.4 
31.0 
30.6 
30.2 
29.8 
29.6 
29.4 
29.7 
30.6 
32.0 
33.7 

29.9 
29.4 

28.9 
28.5 
38.1 
27.9 
27.8 
28.4 
29.7 
31.3 
33.1 
34.6 

:i5.8 

36.9 
37.2 
37.2 
36.5 
35.3 
34.4 
33.6 
32.8 
32.2 
31.6 
31.1 

40.1 
39.4 
38.8 
38.3 
37.9 
37.6 
37.8 
38.8 
40.5 
42.2 
44.0 
45.7 
47.0 
48.3 
48.7 
48.8 
48.0 
47.0 
45.6 
44.5 
43.4 
42.5 
41.7 
40.9 

49.1 
48.3 
47.6 
47.1 
46.5 
46.4 
47.4 
49.5 
51.7 
53.7 
55.6 
57.1 
58.2 
59.0 
59.6 
59.6 
59.0 
57.9 
.56.2 
.54.9 
53.6 
52.5 
51.6 
50.5 

59.6 
58.8 
58.1 
57.5 
57.0 
57.4 
59.0 
61.3 
63.4 
65.2 
67.1 
68.4 
69.5 
70.5 
70.7 
70.7 
70.2 
69.0 
67.0 
65.2 
63.7 
62.6 
61.5 
60.9 

68.0 
67.2 
66.4 
65.8 
65.2 
66.1 
68.2 
70.5 
72.8 
74.6 
76.3 
77.5 
78.7 
79.6 
79.9 
79.8 
78.9 
77.6 
75.7 
74.1 
72.6 
71.3 
70.2 
69.2 

72.9 

72.2 
7l'.4 
70.8 
70.3 
70.6 
72.4 
74.7 
77.2 
79^1 
80.9 
82.4 
83.4 
84.0 
84.2 
84.1 
83.1 
81.8 
80.1 
78.3 
76.6 
75.6 
74.5 
73.7 

71.3 

70.7 
69.9 
69.3 
68.7 
68.8 
70.4 
73.0 
75.4 
77.7 
79.8 
81.0 
82.1 
82.8 
82.9 
82.6 
81.6 
80.5 
78.8 
77.0 
75.6 
74.4 
73.2 
72.2 

65.2 
64.6 
63.8 
63.1 
62.5 
62.2 
63.1 
65.7 
68.0 
70.5 
72.6 
74.2 
75.3 
76.1 
76.4 
76.3 
75.1 
73.4 
71.6 
70.2 
68.6 
67.5 
66.8 
65.7 

53.9 
53.3 
52.7 
52.1 
51.7 
51.3 
51.6 
53.6 
56.0 
58.4 
60.6 
62.3 
63.4 
64.2 
64.4 
64.0 
62.8 
61.2 
.59.5 
58.1 
56.9 
55.9 
55.0 
54.2 

44.1  35.0 
43.7     34.5 

3 

43.2  '  34.2 

i 

43.7  .  33.8 

42.3     as. 4 

6 

42.0     33.0 

41.9     32.9 

8 

9 

42.8  33.3 
44.8  1  34.5 

10 

11                  

46.7  36.0 
48.6     37.9 

Noon 

1 

3.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'. 

4 

5 

6 

8 

9 

35.0 
36.0 
36.8 
37.2 
.37.0 
.36.2 
a5.4 
34.5 
34.0 
33.0 
32.8 
32.1 
31.8 

50.2  39.4 
51.4     40.7 

52.0  41.6 
53.2  42.0 
51.6  41.5 
50.4     40.5 

49.2  39.5 

48.1  :  38.6 

47.3  37.9 

46.4  37.1 

10 

11 

45.7  36.5 
44.9  35.9 
44.2     35.2 

32.9 

32.2 

42.8 

53.0 

63.9 

72.8 

77.3 

75.8 

69.1 

57.4 

46.5     36.9 

MEAN  HOURLY  TEMPERATURE. 

Spring. 

Summer.     |      Autumn. 

"Winter. 

Year. 

1  A.  M 

49.6 
48.8 
48.2 
47.6 
47.1 
47.1 
48.1 
49.9 
51.9 
53.7 
.55.6 
57.1 
.58.2 
59.2 
59.7 
.59.7 
.59.1 
58.0 
.56.3 
.54.9 
53.6 
52.5 
51.6 
50.8 

70.7 
70.0 
69.2 
68.6 
68.1 
68.5 
70.3 
72.7 
75!l 
77.1 
79.0 
80.3 
81.4 
82.1 
82.3 
83.2 
81.2 
80.0 
78.2 
76.1 
74.9 
73.8 
72.6 
71.7 

54.4 
53.9 
53.2 
52.6 
52.2 
51.8 
53.2 
54.0 
56.3 
58.5 
60.6 
62.2 
63.4 
(U.l 
64.3 
64.0 
62.8 
61.3 
59.7 
58.5 
57.3 
56.4 
55.6 
54.7 

32.1 
31.6 
31.2 
30.8 
30.4 
30.3 

ai.o 

30.5 
31.6 
33.1 
34.9 
36.3 
37.5 
38.4 
38.8 
38.6 
37.7 
36.7 

a5.8 

35.2 
34.3 
33.8 
dii.-Z 
33.7 

51.7 

o 

51.1 

3 

4 

50.5 
49.9 

49.4 

49.4 

- 

.50.2 

8 

51.8 

9 

53.7 

10 

55.6 

11 

.59.0 

1 

60.1 

*> 

61.0 

3 

61.3 

4 

61.1 
60.2 

6 

59.0 

57.5 

S 

.56.3 

9 

10 

11 

.55.0 
54.1 
53.2 

52.5 

53.2 

75.3 

.57.7 

34.0 

55.0 

Table  XI  shows  the  mean  temperature  for  each  hour  of  the  day,  based  on 
the  continuous  record  of  a  Richard  thermograph  for  the  ten-year  period 
ending  December  31,  1902.  The  thermograph  record  was  corrected  daily  by 
direct  observations  of  a  mercurial  thermometer  at  8  a.  m.  and  8  p.  m.,  and  by 
the  readings  of  a  maximum  and  a  minimum  self-registering  thermometer. 
The  annual  mean  (55.0°)  is  the  average  value  of  over  87,000  hourly  observa- 
tions, and  may  be  regarded  as  a  true  normal  value  for  the  period  covered  by 
the  observations.  The  same  results  are  graphically  shown  in  Fig.  10,  for 
January,  April,  July,  October,  and  the  year,  and  in  Fig.  11,  for  all  the  months 
of  the  year. 


62 


THE    CLIMATE    OF   BALTIMORE 


ing  a  readier  means  of  observing  the  relative  changes  from  hour  to  hour 
and  the  distribution  of  temperature  within  the  day,  the  month,  and  the 
year.  The  method  of  presentation  employed  in  Fig.  11  is  particularly 
well  adapted  to  a  rapid  survey  of  the  hourly  and  seasonal  distribution. 
In  construction  it  resembles  the  maps  prepared  for  showing  the  varying 
topography  of  an  area.  In  place  of  the  meridians  of  longitude  and  par- 
allels of  latitude  to  arrive  at  the  geographical  position  of  a  given  locality, 
we  have  vertical  lines  to  represent  the  hours  of  the  day  and  horizontal 
lines  for  the  months  of  the  year,  the  intersection  of  which  gives  us  the 


Fig.   11. — Isopleths  of  Hourly  Temperature. 

Fig.  11  shows  the  average  distribution  of  temperature  throughout  the  day  and  j  ear,  based 
on  observations  of  ten  years  of  hourly  readings  of  the  thermograph.  The  hours  of  the  day 
are  indicated  by  the  upper  line  of  figures,  Avhile  the  marginal  letters  indicate  the  months 
of  the  year.  The  line  enclosing  the  area  of  lightest  shading  defines  the  time  of  occurrence 
of  the  lowest  temperature  of  the  year  and  day ;  rise  in  temperature  is  indicated  by  increase 
in  the  intensity  of  .shading.  The  diagram  indicates  that  the  lowest  temperatures  of  tlie  year 
occur  in  the  early  morning  hours  of  January  and  Februarj',  and  that  the  highest  occur  in 
the  eai'ly  afternoon  hours  of  Julj%  based  on  the  average  of  a  long  series  of  years.  The 
curved  lines  show  the  liours  of  the  day  and  the  months  of  the  year  when  the  average  read- 
ings of  the  thermometer  are  equal :  these  lines  are  called  chrono-isotherms,  or.  isopleths  of 
temperature.  The  dotted  lines  marlicd  S.R.  and  S.S.  show  the  time  of  sunrise  and  sunset. 
See  also  Table  Xi. 


time  sought.  In  place  of  the  contour  lines,  or  lines  of  equal  elevation 
of  the  topographic  map,  we  have  lines  of  equal  temperature  (or  isopleths 
of  temperature  as  they  are  called  when  used  in  this  manner)  projected 
upon  the  plane  of  the  time  lines.  The  rapid  detection  of  the  diurnal 
and  annual  distribution  of  temperature  is  further  facilitated  by  means  of 
a  system  of  shaded  areas,  increase  in  the  intensity  of  the  shade  signifying 


:MARYLAXD   "WEATHER    SERVICE  63 

increase  in  temperature.  Consulting  Fig.  11  we  find  that  for  Baltimore 
a  temperature  of  8-i°  is  limited,  under  average  conditions,  to  the  hours 
from  2  p.  m.  to  4  p.  m.,  during  the  month  of  July;  that  the  temperature 
of  75°  occurs,  on  the  average,  from  June,  between  the  hours  of  10  a.  m.  and 
7  p.  m.,  to  September,  between  1  p.  m.  and  5  p.  m.,  etc.  In  the  winter 
months  the  line  of  32°,  or  freezing  weather  for  example,  is  limited,  on 
the  average,  to  the  months  of  January  and  February  from  midnight  to 
10  a.  m.  We  see  that  the  average  summer  temperature  of  76°  extends  from 
June  to  September  during  the  middle  hours  of  the  day,  while  the  average 
winter  temperature  of  36°  is  confined  to  the  night  and  morning  hours  of 
December,  January  and  February,  and  to  a  few  of  the  early  morning 
hours  of  March.  The  lowest  temperature  of  the  day  occurs,  on  the  aver- 
age, just  before  sunrise  and  hence  varies  with  the  advance  and  retreat  of 
the  sun.  The  time  of  occurrence  of  the  highest  temperature  varies  less 
with  the  season,  occurring  throughout  the  year  between  3  p.  m.  and  4 
p.  m.,  excepting  the  month  of  November  when  the  highest  temperature 
of  the  day  occurs  at  about  2.30  p.  m. 

The  diurnal  variation  of  temperature  is  represented  by  a  simple  curve 
which  rises  steadily  from  a  minimum  point  just  before  sunrise,  attains  a 
maximum  in  the  early  afternoon  hours,  and  then  descends  without  inter- 
ruption to  the  early  morning  minimum.  In  this  respect  it  differs  from 
the  curve  representing  the  diurnal  variation  of  the  barometer  which, 
as  we  have  seen,  has  a  double  period,  Ts-ith  primary  and  secondary  maxi- 
mum and  minimum  points. 

Phases  of  the  Diurnal  Variation. 

The  principal  phases  of  the  diurnal  variation  of  temperature  are  pre- 
sented in  Table  XII,  containing  a  summary  of  the  average  time  of  occur- 
rence of  the  minimum,  the  maximum,  and  the  mean  temperature  for  the 
day,  and  the  varying  interval  between  the  occurrence  of  the  minimum 
and  maximum  points.  In  the  months  of  May  and  June  the  lowest  tem- 
perature of  the  day  occurs  at  5.05  a.  m.,  75th  meridian  time,  which  is 
six  minutes  faster  than  Baltimore  local  time;  the  time  advances  steadily 
to  6.50  a.  m.  in  January,  returning  again  to  5.05  a.  m.  in  May.  Tlie 
maximum  of  tlio  day  sliows  less  variation  in  time  of  occurrence.     From 


64 


THE    CLIMATE    OF    BALTIMORE 


January  to  May  it  remains  at  about  3.25  p.  m.,  then  occurs  successively 
earlier  in  the  day  until  November,  when  a  maximum  is  attained  at  2.25 
p.  m.  It  seems  rather  remarkable  that  the  maximum  temperature  of 
the  day  should  appear  earliest  in  the  month  of  November.  The  average 
temperature  occurs  first  at  about  9  a.  m.  in  the  summer  months  and  at 
10.30  a.  m.  in  the  winter  months,  and  again  between  8.30  p.  m.  and  9.00 
p.  m.  in  summer,  and  about  10.00  p.  m.  in  the  winter  months,  excepting 


v^ 

"-^ 

"~~~ 

— " 

Max 

.. 









1.  Mean 



, 

■ 

■— ^ 

MiN. 

■ 

_,^ 

-^^ 



_ 

Fig.  12. — Principal  Phases  of  Diurnal  Variation  of  Temperature. 

Fig.  12  shows  the  time  of  occurrence  of  the  highest  and  lowest  points  indicated  by  the 
thermometer  on  an  average  day  for  each  month  ;  also  the  morning  and  afternoon  hours 
when  the  mean  temperature  of  the  day  is  most  likely  to  occur.    See  also  Table  XII. 

December,  when  it  occurs  as  early  as  9.20  p.  m.  The  amplitude  of  varia- 
tion, or  the  difference  between  the  daily  maximum  and  daily  minimum 
temperature,  is  greatest  in  the  month  of  June  (14°. 7)  and  is  smallest 
in  the  month  of  Januarv^  (7°.0) .     (See  Fig.  12.) 

The  temperatures  thus  far  discussed  are  average  values  for  a  period 
of  ten  years.  When  we  examine  into  the  time  of  occurrence  of  the  prin- 
cipal phases  of  the  diurnal  march  of  temperature  more  closely  we  find 


MARYLAND    WEATHER    SERVICE 


65 


a  wide  divergence  from  the  average  time  of  occurrence  as  recorded  in 
preceding  paragraphs.  The  limits  of  variability  in  the  average  time 
for  a  single  month  are  shown  in  the  following  tabular  statement  contain- 
ing the  hour  and  the  frequency  of  occurrence  of  each  phase  in  each 
month  of  the  ten-year  period. 


TABLE  XII.— TEMPERATURE  PHASES. 


Minimum. 


Fre- 
quency. 


5'  6  7  8 


January  

February . .   

March 

April  

May 

June 

July 

August 

September ]  2 

October 

November I  1 

December 


Tear. 


4  9 


4  4 


6:50 
6:30 
6:15 
5:«) 
5:05 
5:05 
5:10 
5:25 
5:50 
6:05 
6:25 
6:40 


5:50 


1st  Mean. 


Fre- 
quency. 


9  10    11 


5 
5 

6 

3  7 
7  3 
9     1 

9     1 

9|  1 
4|    6 

4  6 
l|    8 

6 


5     5 


9:40 


Maximum. 


Fre- 
quency. 


p.  m. 


1  2  3i  4  5 


7  5 

5  4 

41 

6!  5 

7  4 

21  5 


9  2 


II  4. 


3:10 


2nd  Mean. 


Fre- 
quency. 


p.  m. 


8!  91011 


1  9'.. 

2  8., 

3  7., 


ih-m. 


8  4 


1  9:50 
3  10:00 
9:40 
9:30 
8:50 
8:50 
8:35 
8:50 
8:40 
8:35 
8:55 
9:20 


8-30 
9-00 
9-10 
9-50 
10-20 
10-05 
9-55 
9-20 
9-00 
8-35 
8-00 
8-20 


9:00     9-20 


Table  XII  indicates  the  average  time  of  occurrence  of  the  lowest  and 
highest  temperature  of  the  day  for  each  month  and  for  the  year;  the  morning 
and  the  afternoon  hours  when  the  mean  temperature  of  the  day  is  most 
likely  to  occur;  the  frequency  of  occurrence  of  these  phases  at  given  hours; 
and  the  average  number  of  hours  between  the  occurrence  of  the  highest  and 
lowest  temperatures  of  the  day.  The  values  are  based  on  hourly  observations 
for  the  ten-year  period  from  1893  to  1902.     See  also  Fig.  12. 


The  January  minimum  may  occur  from  5  a.  m.  to  8  a.  m.,  the  normal 
time  being  6.50  a.  m.  In  May,  June,  and  July  the  minimum  occurs 
with  great  regularity  at  about  5  a.  m.,  and  in  September  and  October  at 
6  a.  m.  There  is  more  uniformity  in  the  time  of  occurrence  of  the  max- 
imum temperature  of  the  day;  this  does  not  vary  greatly  from  the  hour 
of  3  p.  m.  at  any  season  of  the  year.  The  earliest  occurrence  of  the 
maximum  is  in  the  month  of  November.     The  time  interval  between  the 


66  THE    CLIMATE   OF    BALTIMORE 

minimum  and  maximum  of  the  da}'  increases  steadily  from  the  winter 
months  to  the  summer  months.  Beginning  with  eight  hours  and  thirty 
minutes  in  January,  the  interval  reaches  a  maximum  in  May  when  it 
amounts  to  ten  hours  and  twenty  minutes,  then  decreases  regularly  to 
eight  hours  in  November.  This  difference  in  time  is  due  mostly  to 
variations  in  the  time  of  occurrence  of  the  minimum  temperature. 

DiuRXAL  Variation  as  Affected  by  Clouds  and  Eain. 

In  considering  the  diurnal  variation  of  temperature  in  the  preceding 
paragraphs  the  character  of  the  day  does  not  enter  into  the  problem. 
The  average  values  given  include  all  days  for  a  period  of  ten  years.  The 
amplitude  of  variation  of  temperature  is  manifestly  largely  dependent 
upon  the  presence  or  absence  of  clouds.  On  a  cloudy  day  the  sun's  rays 
are  largely  absorbed  by  the  cloudmass  and  comparatively  little  of  the 
sun's  heat-rays  reach  the  earth's  surface  directly.  To  discover  to  what 
extent  the  normal  daily  variation  is  affected  by  the  character  of  the  day 
the  diurnal  variation  of  temperature  has  been  determined  for  selected 
days  in  each  season.  For  this  purpose  the  days  were  grouped  as  clear, 
cloud}'',  and  rainy.  Days  were  regarded  as  clear  during  which  the  per- 
centage of  sunshine  exceeded  90  per  cent  of  the  possible  amount  for  the 
day.  They  were  considered  cloudy  when  the  sky  was  overcast  the  entire 
day.  A  day  was  considered  rainy  when  rain  fell  for  more  than  four 
hours,  not  necessarily  consecutive.  Each  group  included  approximately 
100  days,  selected  from  all  seasons  of  the  year.  A  further  restriction 
was  imposed  by  excluding  days  with  a  moderate  or  a  high  wind,  as  it 
was  desired  to  eliminate  the  effect  of  wind  velocity  upon  the  diurnal 
variation  in  this  problem. 

The  results  of  the  above  classification  are  shown  in  Fig.  13,  in  which 
some  interesting  and  instructive  relations  are  revealed.  The  ampli- 
tude, or  difference  between  the  lowest  and  highest  temperature  of  the 
day,  is  manifestly  greatest  on  clear  days,  with  a  maximum  in  the  spring 
months.  Cloudiness  reduces  the  daily  range  of  temperature  to  less  than 
one-half  of  that  on  a  clear  day.  On  a  rainy  day  the  difference  between 
the  maximum  and  minimum  is  reduced  to  2°  or  3°,  equivalent  to  about 
one-fourth  the  range  on  a  clear  dav  in  winter  and  to  about  one-sixth 


MARYLAXD    WEATHER    SERVICE 


67 


the  range  in  spring,  summer,  and  autumn.  The  principal  phases  of  the 
diurnal  march  of  temperature  do  not  materially  change  in  the  summer 
and  autumn  months.  The  minimum  occurs  approximately  at  sunrise 
and  the  maximum  of  the  day  in  the  early  afternoon  hours.  There  is  a 
marked  deviation,  however,  from  the  normal  conditions  on  rainy  days 
in  autumn  and  winter.     After  attaining  the  maximum  for  the  dav  it  is 


1    2 

(    4     ■ 

« 

1 

i 

> » 

9    1 

uoa 

_ 

_ 

♦ 

> 

r 

i 

9 

9 

1  r 

2 

1 

-    -        -    ' 

I 

1 

y 

\\ 

■  1 

/ 

/ 

\  \ 

\1. 

1 

/ 

/ 

\ 

1 

Ov^v 

\ 

1   ^^ 

:jtjL 

\   '  "^ 

/'    y 

n: 

Sf. 

/I  ''  / 

■  \  -1. 

'~ 

r^ 

i/ 

iS 

^H 

1 

i  \f 

- 

1 

-- 

-- 

- 

- 

- 

7- 

- 

- 

- 

- 

- 

- 

- 

- 

- 

■t\ 

50 

/ 

i» 

w 

II   L 

t' 

, 

. — ' 

' 

~~ 

p> 

-^ 

Ft 

°^--C^*fe-/ 

<>. 

//- 

;  '  '  -i~-r 

1 

\ 

■  ;  ,  :y^,   1 

i 

\->        1   /i    :    1 

Sp 

ri 

h 

iS 

^■':/-]-V-\ 

[_ 

N^y,   i   !   1 

^ 

rn  1  ! 

1 

1 

y 

\ 

/ 

/ 

\ 

'  i 

/ 

-^ 

~H 

-V, 

N 

i  '  U 

V 

■ 

-^ 

\ 

A 

mr 

i^jL 

1 

- 

- 

- 

-- 

- 

--- 

/i-< 

- 

- 

- 

- 

-- 

- 

'^ 

c 

/ 

/ 

\  ^■^f' 

ii 

^ 

\ 

\ 

^ 

> 

k'«a 

it- 

'^ 

\ 

50 

s 

1 

\ 

S' 

1 

s 

1 

1 

II 

II 

m 

n 

1   1 

V 

^ 

1   1 

_ 

2 

J 

5 

( 

9 

10 

K 

on 

J    ■! 

J    < 

r 

s 

i 

o 

1    12 

i    i    '    i    1    !    1    :  >^ 

i  ■  '  1  M 

i  y' 

■  1 

s 

85 

y 

1 

^ 

! 

/ 

! 

1 

1 

\ 

/ 

/ 

/ 

, 

7 

j\f-i. 

1 

^ 

75 

J  ]   1 

/' 

\                     \ 

y 

1 

/ 

" — ^^                 N 

/ 

1 

ft 

/ 

1       ^^"^  J 

<5 

^' 

^ 

,t^ 

V 

!           ;  ^^        ! 

jSS" 

.L^.JAj  . 

_j  ^     lJ  Xi  I 

70 

N^'  :/  ,  1  < 

'  '            \ 

'  ^- — -^'          \l 

X 

s 

^u 

mmei-       V 

< 

^ 

\ 

/ 

r-"  1 

1 

' 

i=?- 

\r 

\;_^>^ 

_L 

■      1 

,    i'       1 

dh 

38 

— 1  1  1  1  1  M 

\  • 

1 

— '_  \ 

^ 

— 

— 

— 

-^ 

1    1    :    .    1    ■ 

:   ') 

35 

r,:^^^^,^...-.-/-.. 

^ 

h 

-  -  - 

-^ 

^ 

/ 

s 

i    1 

1/ 

\ 

^t^ 

1.  1 

■V,    1 

y 

■ 

V 

^J 

^'  J 

1 

/ 

N 

N 

5-1 

t 

V 

^\^^ 

f» 

. 

s 

f 

/ 

23 

" 

:_ 

Fig.   13. — Effect  of  Cloudiness  and  Rain  on  the  Hourly  Variations  of  Temperature. 


maintained  until  nearly  midnight.  One  of  the  most  interesting  facts 
revealed  in  the  diagrams  is  the  relative  position  of  the  curves  for  clear, 
cloudy,  and  rainy  days  in  the  difTerent  seasons.  In  winter  the  clear  day 
has  a  temperature  decidedly  below  the  normal  for  the  season,  while  the 
cloudy  and  raiiiv  (Uiys  are  well  al)ovo  tb.e  normal.  In  spring  the  clear 
day  lias  altout  the  normal  temperature:  the  cloudy  day  is  far  above  tbe 
normal :  the  raiin-  dav  is  decidedlv  below  tlie  normal.    In  summer  the  clear 


68 


THE    CLIMATE    OF    BALTIMORE 


day  is  decidedly  warmer  than  the  average  for  the  season ;  the  cloudy  day 
is  about  normal;  the  rainy  day  is  much  below  the  normal.  In  autumn 
the  clear  day  is  somewhat  below  the  average  temperature,  the  cloudy  day 
is  about  normal,  and  the  rainy  day  is  well  above  the  normal.  These  dif- 
ferences in  temperature  depending  upon  the  extent  of  cloudiness  and  pre- 
cipitation are  in  some  cases  very  large.  In  spring  the  early  morning 
temperatures  may  be  10°  to  15°  lower  with  a  clear  sky  than  with  an 
overcast  sky.  In  autumn  there  is  quite  as  marked  a  difference  between 
a  clear  and  a  rainy  day  in  the  early  morning  hours.  In  the  summer 
months  the  midday  temperatures  may  be  reduced  10°  to  13°  by  an  over- 


s' 


30* 


Fig.   14. — Effect  of  Snow-Covering  on  the  Hourly  Variations  of  Temperature, 
(a)  A  normal  winter  day. 
{b)  Average  of  days  with  snow  on  the  ground. 

cast  sky,  and  15°  to  20°  during  a  rain.  During  the  autumn  an  overcast 
sky  will  maintain  the  average  temperature  of  the  day  6°  to  8°  above 
that  of  a  clear  day. 

MEAN  HOURLY  TEMPERATURE  ON  CLEAR,  ON  CLOUDY  AND  ON  RAINY  DAYS. 


Winter. 


Normal  Temperature 34.0° 

Clear  days  (Departures) —5.2° 

Cloudy  days         "            i  +0.6° 

Rainy  days          "             +0.5° 


Spring. 


53.2° 
1 .70 

+3.5° 
-6.2° 


Summer. 


75.8° 

+3.4° 
4  2° 

-:!o° 


Autumn. 


57.7': 


-0.4° 
+0.9° 


ilARYLAXD    WEATHER    SERVICE 


69 


Effect  of  a  Sxow  Coverixg. 

To  determine  the  effect  of  a  snow  covering  upon  the  diurnal  variation 
of  temperature  the  average  hourly  temperature  was  calculated  for  all 
da3's  within  the  period  of  ten  years  from  1893  to  1902  upon  which  the 
ground  was  covered  with  snow  to  a  depth  of  half  an  inch  or  more.  The 
values  for  the  entire  season  are  shown  in  the  accompanying  table  and  in 
Fig.  14  in  comparison  with  the  normal  temperatures  for  the  winter 
season.  The  two  curves  are  identical  in  form  and  run  parallel  through- 
out their  extent,  but  the  days  with  snow  on  the  ground  were  uniformly 
about  10°  below  the  normal  temperature  for  the  winter  months. 


HOURLY  TEMPERATURES  ON  DAYS  WITH  SNOW  ON  THE  GROUND. 


Hours:  A.  M. 

1 

2 

3 

4 

5 

6 

7 

8 

9        10 

11 

Noon. 

Winter  normal.. 
With   snow  on 

ground 

Departure  below 

normal   

32.1 
21.8 
10.3 

31.6 
21.2 
10.4 

31.2 
20.8 
10.4 

30.8 
20.4 
10.4 

30.4 
20.0 
10.4 

30.2 
19.5 
10.7 

30.0 
19.3 
10.8 

30.5 
19.5 
11.0 

31.6  33.1 

20.7  32.6 
10.9    10.5 

34.9 
24.6 
10.3 

36.3 
26.0 
10.3 

Hours:  P.  M. 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

Mid- 
night. 

Means. 

Winter  normal.. 
With   snow  on 

ground    

Departure  below 

normal 

37.5 
27.3 
10.2 

38.4 
28.3 
10.1 

38.8 

28.9 

9.9 

38.6 

28.9 

9.7 

37.7 

27.8 

9.9 

36.7 

26.8 

9.9 

35.8 
25.7 
10.1 

35.2 
24.9 
10.3 

34.3 
24.0 

10.3 

33.8 
23.4 

10.4 

33.2 
22.5 
10.7 

32.7 
23.0 
10.7 

34.0 
23.6 
10.4 

The  temperature  is  lowered  during  the  night  by  the  intenser  radiation 
from  a  snow  surface ;  it  is  prevented  from  rising  during  the  day  because 
much  of  the  heat  of  the  sun  which  would  otherwise  go  to  warm  tlie 
atmosphere  is  spent  in  melting  and  vaporizing  the  snow.  The  air  tem- 
perature is  likewise  reduced  by  the  snow  preventing  the  communication 
of  heat  from  the  ground  by  convection.  As  observations  show  that  tlie 
difference  between  the  normal  hourly  winter  temperature  and  the  hourly 
temperature  over  a  snow-covered  ground  is  practically  constant  through- 
out tlie  day  and  night,  the  daily  range  is  neither  increased  nor  decreased 
by  the  presence  of  snow.  The  low  average  temperature  of  the  winter  of 
l!Hi;;-l!J01  was  doul)tlos.s  largely  due  to  the  exceptional  d\iration  of  a 
snow  cover.     The  depth  of  snow  was  not  great,  in  the  vicinity  of  Balti- 


70  THE    CLIMATE   OF    BALTIMORE 

more,  but  a  moderate  snow  covering  persisted  during  a  period  of  time 
nearly  double  the  usual  length.  There  is  some  compensation  in  the 
beneficial  protection  afforded  by  snow  to  winter  wheat  and  to  vegetation 
in  general  by  preventing  the  penetration  of  frost  into  the  ground. 

The  Effect  of  Wixd  Velocity  ox  Temperature. 

Another  factor  which  largely  affects  the  diurnal  range  of  the  ther- 
mometer is  the  movement  of  the  atmosphere.  It  is  well  known  that  in 
a  quiet  atmosphere  there  may  be  a  great  difference  in  temperature  at  the 
earth's  surface  and  a  small  distance  above.  In  the  night  and  early  morn- 
ing hours  of  winter  the  thermometer  may  register  5°  or  10°  lower  near 
the  ground  than  on  the  house  tops ;  on  a  hot  summer's  day  the  difference 
at  midday  may  be  quite  as  large  but  reversed.  In  either  case  the  lower 
layers  of  the  quiet  atmosphere  tend  to  take  on  the  temperature  of  the 
ground.  Such  differences  are  particularly  common  in  the  lower-lying 
portions  of  any  locality.  The}''  do  not  occur  in  an  active  atmosphere;  a 
breeze  will  quickly  level  any  marked  differences  in  the  temperature  of 
any  neighboring  strata  of  air  by  intermingling  of  the  lower  and  higher 
layers  resulting  in  an  approximately  uniform  temperature.  The  effect 
of  wind  movement  on  the  diurnal  range  of  temperature  may  be  clearly 
shown  by  classifying  a  large  number  of  days  according  to  total  daily 
wind  movement,  days  which  in  other  respects  have  approximate!}^  similar 
conditions.  The  results  of  such  a  classification  are  graphically  shown  in 
Fig.  15.  An  equal  number  of  clear  or  approximately  clear  days  was 
selected  in  each  of  the  months  of  January,  March,  July,  and  October. 
Those  having  a  total  daily  wind  movement  of  less  than  100  miles 
per  day  were  placed  in  one  group;  another  group  contained  days  with  a 
total  daily  wind  movement  between  200  miles  and  300  miles;  still  an- 
other group  comprised  winter  and  summer  days  with  a  wind  movement 
exceeding  400  miles  per  day.  The  average  hourly  temperature  was  then 
determined  for  each  group  separately  and  a  comparison  made  between  the 
resulting  temperatures.  In  each  case  the  diurnal  range  of  temperature  is 
seen  to  be  markedly  lower  with  increase  in  wind  movement.  In  the 
following  tabular  statement  tlie  total  daily  range  for  each  condition 
mentioned   above   is   given,   while   in   the  .succeeding   table   the   hourly 


MARYLAND    WEATHER    SERVICE 


71 


3         6         9     Noon      3         6         9      Mot. 


3         6         9      Noon      3         6         9      Mot 


Id 

'    '  '  1 

i  :  M  ' 

1  '    '  ! 

'    , 

Ml 

1    j 

y-^-, 

'    ■    1    ' 

i     '  1 

1    ,    ■ 

a        X 

(  '  1  1 

1    1       ' 

/    ;    i         ^ 

'Mil 

V 

.  '  I  1 

tV 

■  1  i 

1  ]     / 

X. 

,  '     ' 

1  1   / 

1    1 

!  '  Xh>. 

' 

/ 

'  ' 

,  '  / 

I  ;  ;  'i-! 

1   !   '   1    1 

/ 

1  '  i  ! 

°    "V 

'   1 

Si;: 

1 . '/ 

,      > 

;      '      '      ■ 

^' 

/  / 

x'  i  [ 

^/v 

/  / 

^  , ^ 

N 

>v^. 

/       >> 

,  xj  ■ 

i  v 

.  i>> 

s\  rv- 

4 

L/ 

Sill 

X 

X\ 

[ 

xj 

;  V. 

\\^\ 

1 

x^ 

-r 

,>V4^ 

' 

'    ' 

' 

'     1  ' 

■ 

'   j 

;     ;     ' 

11     1 

[ 

t       1 

in 

•"X 

1  1  j 

/•^    \ 

'     1         1 

"SO 

1  '      ; 

^\^-r^\^ 

1              \ 

'  '  1 

'  :  :    > 

\           '     ' 

VV 

X    '. 

,  (  '  /  / 

[ 

,\^ 

X, 

i     // 

--^j 

NX 

X 

IX     / 

'    ^^''**^ 

/  !  !      ' 

'^S*. 

— .- 

; 

/     / 

i  1  1  '  1 

'  1 

/'    / 1 

'  /    / 

, 

'       '  X  ' 

'^'^^'  1 

1 

1     j     1  V 

<X  1  /    1 

1  1 

!  1 1 1 

N  ,   ; 

/i  ! 

1 

x;^ 

/ 1 1 ; 

1 

; 

i   ; 

t 

1 

1    i    ! 

1 

,    ;    i    1    i 

■"' — 'n~ 

I 

11'' 

'  1  i  i  ! 

'•       1    ' 

1 

1 1 

i 

.a<TT 

'     "^  v* 

'       1 

' 

] 

■    ' 

/    .  '\ 

!  i     X 

;  1  : 

■ 

■    ,    V 

^  J  ;  ■  '  ; 

'    ' 

1   '      i 

\   1   ■   1 

■ill 

X 

.   i       1 

i  '  ! 

1^1 

X      1 

!          '     • 

X  ' 

1      1 

/ 

'  '  !  1  ' 

Xi 

'   1 

!  1  ■  ■  ! 

/  ;    1 

:  '  I     I 

I 

1  i :  X 

i          1 

'      '    [    ' 

t 

■^   '  1 

■   1 

/ 

--^    '    j 

X-c 

.   ,   1 

i  1  i  i  ' 

/             ; 

''    ■  1 

■^  "=2 

,  '**. 

'   1   '   ' 

1        i 

,  :  X 

'  '  •  1 

■"  4    h 

r      '      '           i 

' 

i  1  '  1  ' 

!     / 

.X\ 

;    '    '    ! 

, 

X 

;      '            .      I 

/ 

X 

i  ;  '  1 

X: 

' 

/ 

/  :  ■ 

\\\\ 

,   V 

.j"^*.^ 

,     1     j     J 

y 

/     ■  •  \ 

^^ 

"""^^^n^ 

"y-             -^ 

>X/ 

\ '  \  \  \ 

'    !    1 

'      '      "^^ 

■  ' 

'    ,  1  [ 

'    , 

' 

i  1  M  : 

'  1  1  '  i 

1 

■  I    1 

1    ; 

1      '      1      ' 

*  .  .. 

.     .     1     .     : 

-cr='=^ 

;  '  j 

^    ,      , 

•    '    ;    !    ' 

.... 

'     '     \^ 

'  'lit 

-4'  '  '  '  ' 

*  '  I  !  • 

TX 

1 

iX 

i    ' 

;  XL 

'     1  ' 

""""^S^     ' 

■^ 

'Ml 

i  j  1 1  , 

TX- 

■  ' : !  i ; 

'         ,    1 

'^  1  "i  '-^~. 

1  1  <  1  ; 

i 

M  , 

X-Li^ 

EAR 

3         6         9      Noon 


3         6         9      Noon      3         6         9      Mo' 


Fig.   15.  — Effect  of  Wiud  Velocity  on  tlie  Hourly  Variations  of  Temperature. 
(a)  On  days  with  a  light  wind. 
ib)  On  days  with  a  moderate  wind. 
(c)  On  days  with  a  high  wind. 


72 


THE    CLIMATE    OF   BALTIMORE 


changes  are  shown  for  the  year,  expressed  in  terms  of  departures  from 
the  normal  temperatures  for  the  year. 

Eange  of  Temperature  on  Calm  axd  AYixdt  Days. 

Total  daily  wind  movement.              Jan.              March.              July.              Oct.  Year. 

Less  than  100  miles 14.6°         15.4°         19.5°         18.9°  16.8° 

From  200  to  300  miles....   5.9°         12.0°         11.0°           8.2°  9.0° 

Over  400  miles Winter  5.2°             Summer  5.7°  5.4° 


hourly  temperature  on  calm  and  on  windy  days. 

(Expressed  in  terms  of  departures  from  the  normal  temperature.) 


Hours  :  A.  M.          1 

J 

2 

3          4          5 

6 

7 

8 

9 

10 

11 

Noon. 

Normal  Temper- 

51.7° 
-  2.6 
-3.6 
-11.1 

51.1° 
-  2.7 
-3.5 
-11.2 

50.5° 
-2.6 
-3.4 
-11.2 

49.9° 
-  2.6 
-3.3 
-11.2 

49.4° 
-2.6 
-  3.2 
-11.3 

49.4° 
-  2.6 
-3.5 
—11.9 

50.2° 

-  2.6 

-  4.6 
-13.2 

51.8° 
-  1.9 
-5.0 
-14.5 

53.7° 

-  1.0 

-  5.4 
-15.7 

55.6° 
+  0.1 
-  5.7 
-16.6 

57.5° 
+  1.0 
—  6.4 
-17.7 

59.0° 
+  1.7 
-  6.2 
-18.1 

50-100  Miles  (De- 
partures)   

200-300  Miles  (De- 
partures)  

Over    400    Miles 
(Departures).. 

Hours :  P.  M. 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

Mid- 
night. 

Means. 

Normal  Temper- 
ature   

50-100  Miles  (De- 
partures)   

200-300  Miles  (De- 
partures)  

Over    400    Miles 
(Departures).. 

60.1° 
+  2.1 
—  6.4 
-18.5 

61.0° 

+  2.1 

-  6.4 

-18.7 

61.3° 
+  2.3 
-6.5 

-18.9 

61.1° 
+  2.2 
-6.4 
-19.1 

60.2° 
+  2.1 
-  6.4 
-19.2 

59.0° 
+  1.6 
-6.3 
-18.6 

57.5° 
+  1.7 
-  6.3 

-18.1 

56.3° 
+  1.2 
—  6.2 
-17.5 

55.0° 
+  1-1 
-  6.0 

-17.0 

54.1° 
+  1.1 
-6.0 
-16.7 

53.2° 
+  1.0 
-6.0 
—16.2 

52.5° 
+  1.1 
-6.1 
-16.1 

55.0° 
+  0.1 
—  5.3 
—15.8 

EeDUCTION   TO    THE   TrUE   MeAN    TEMPERATURE. 

As  it  is  often  inconvenient  or  impossible  to  make  daily  observations 
of  the  temperature  at  the  hours  best  suited  to  the  purpose  of  securing  an 
accurate  average  value,  it  is  desirable  to  know  the  corrections  to  be 
applied  to  any  selected  combination  of  hours  in  order  to  arrive  at  a  true 
average  value  for  the  day  for  a  given  locality.  This  can  readily  be  done 
whenever  hourly  observations,  or  continuous  records,  have  been  main- 
tained somewhere  within  a  hundred  miles  or  so  of  the  locality,  pro- 
vided the  physiographical  conditions  of  the  two  localities  do  not  differ 
widely  from  one  another.  In  the  following  table  the  necessary  correc- 
tions have  been  computed  for  the  horizon  of  Baltimore  for  some  of  the 


ilARYLAXD    WEATHER    SERVICE 


combinations  of  hours  of  observation  employed  at  different  times  within 
the  State  of  Maryland. 

CORRECTIONS    TO    REDUCE    OBSERVED    TEMPERATURES    TO    THE    TRUE 

DAILY  MEAX. 


Hours  of  Observation :  —  (75th 
Meridian  Time.) 


J  (7:37  a. +  4:37  p.  +  11:37  p.). . .. 
4(7:00  a.  +  2:00  p.  +  9:00  p.).... 
i  (7:00  a.  +  2:00  p.  +  3(9:00  p.) . . . . 
i  (7:00  a.  +  3:00  p.  +  11:00  p.) . . . . 

J  7:00  a.  +  3:00  p.  +  10:00  p.) 

A  (10:00  a.  +  10:00  p.) 

J  (Maximum  +  Minimum) 

H8:00a.  +  8:00  p.) 

i(Ta.  +  lla.+3p. +  7p. +  llp.) 


+0.2+0.1+0.1+0.1  0 
—0.3—0.3—0.3—0.3—0.5 
—0.3—0.4—0.4—0.4—0.3 


0        0+0.1+0.1 


-0.3-0.2 


+0.5 

0.4 

+1.1 

-0.5 


-0.2 
+0.4 
-0.4 
+1.3 


+0.4 

-0.3 

+1 

-0.6;-0. 81—1.1 


-0.3 
-0.1 


+0.8 


+0 
-0.3 
0 
+0.1 
+0.7 
-1.2 


0 
0.7 
0.4 
0 
0.3 
-0.2 
+0.2 
+0.5 
-1.3 


+0.21+0.3 


-0.5 
-0.3 
+0.3 
-0.1 
-0.2 


0.4 
-0.1 
+0.3 

0.1 
-0.1 

+0.1  0-0.3 
+0.8+1.3+1.1 
-1.1-1.2—1.0—0 


+0.3 
-0.2 
0 
+0.3 
+0.1 
+0.1 


+0.5 
-0.2 
0 
+0.4 
+0.1 
+0.2 
0.4 
+1.6 


+0.5 
-0.3 
0.2 
+0.3 
-0.1 
+0.3 
-0.5 
+1.5 


+0.3 
-0.3 
-0.3 
0 
-0.2 
+0.7 
-0.5 
+1.3 


-0.6-0.6 


0.3 

0.4 
-0.3 
+0.1 

0.2 
+0.3 

0.4 
+  1.0 
-0.9 


In  a  system  of  three  hours  of  observation  the  combination  7  a.m.,  3  p.  m. 
and  11  p.  m.,  gives  a  mean  value  very  close  to  the  24-hour ly  mean,  the 
annual  average  differing  from  the  latter  by  only  0.1°,  while  the  maximum 
departure  is  but  +  0.4°  during  the  month  of  widest  divergence.  During 
four  months  of  the  year,  namely,  January,  February,  June  and  December, 
no  corrections  need  be  applied.  One  of  the  best  combinations  of  two  hours 
is  that  of  10  a.  m.  and  10  p.  m.,  which  yields  an  average  but  0.2°  above 
the  true  annual  mean.  The  maximum  and  minimum  readings  of  self- 
registering  thermometers  require  a  correction  of  —  0.4°  to  the  annual 
average.  Considering  the  great  convenience  of  one  observation  a  day  over 
two  or  more  and  the  further  advantage  of  showing  the  highest  and  lowest 
temperatures,  this  is  the  most  desirable  system  to  adopt.  The  United 
States  Weather  Bureau  maintains  an  organization  of  about  3500  co- 
operating voluntary  observers,  all  reporting  daily  maximum  and  minimum 
temperatures. 

The  Hourly  Eate  of  Change. 

While  the  temperature  increases  steadily  from  sunrise  to  about  3  p.  m. 
and  then  steadily  decreases  to  sunrise,  the  rate  of  warming  and  cooling 


74 


THE    CLI^[ATE   OF    BALTIMORE 


has  its  own  period  which  differs  from  that  of  tlie  temperature  itself. 
The  temperature  rises  most  rapidly  from  8  a.  m.  to  10  a.  m.,  depending 
upon  the  season  of  the  year,  and  falls  most  rapidly  from  6  p.  m.  to  7  p  m. 
The  hours  of  least  change  coincide  with  those  in  whieh  the  maximum 
and  minimum  temperatures  of  the  day  occur.  In  selecting  a  combination 
of  hours  for  observation  it  is  important  to  bear  in  mind  this  varying  rate 
of  change,  and  to  avoid  as  far  as  practicable  the  hours  of  maximum  rate. 
Consideration  of  this  point  is  of  no  consequence  when  maximum  and 


Fig.   16.  —  Hourly  Rate  of  Change  of  Temperature. 

Fiif.  16  shows  the  extent  of  change  in  the  temperature  from  hour  to  hour  throug-hout  the 
day  and  year.  Tlie  values  are  based  on  hourlj'  records  during  a  period  of  ten  years.  The 
houi  s  of  the  day  are  indicated  liy  the  upper  horizontal  line  of  figures,  and  the  months  of  the 
year  by  the  marginal  letters.  The  areas  without  shading  show  the  time  of  day  when  the 
change  in  temperature  is  least,  the  heavj-  black  line  within  this  area  marking  the  time  of 
change  from  falling  to  rising,  or  rising  to  falling  temperature.  The  areas  with  darkest 
shading  show  the  time  of  most  rapid  change.  A  falling  temperature  is  designated  by  a 
minus  sign,  a  rising  by  absence  of  sign,  before  the  figure  representing  the  amovnit  of  change 
in  degrees  and  tenths.  The  dotted  lines  marked  S.R.  and  S.S.  show  the  time  of  sunrise  and 
sunset.  The  time  of  most  rapid  rise  in  the  temperature  is  between  8  a.  m.  and  9  a.  m.,  the 
time  of  most  rapid  fall  is  between  7  p.  m.  and  8  p.  m.    See  Table  XIII  and  Fig.  IT. 


minimum  thermometers  are  employed,  or  when  a  continuous  record  of  the 
temperature  is  maintained.  The  approximate  time  at  wdiich  the  rate  of 
change  is  greatest  and  least  for  each  month  and  for  the  year  is  shown 
below  in  connection  with  the  hours  of  maximum  and  minimum  temper- 
ature of  the  dav. 


MARYLAXD    WEATHER    SERVICE 


VO 


TIME  OF  DIURXAL  MAXIMUM  AND  MIXIMUM  RATE  OF  WARMIXG  AND 

COOLIXG. 


c 

i 

o 

3:30 

>> 

S 

3:.30 

05 

a 

3 

3:00 

< 
3:00 

P. 
® 
t/J 

o 

6 

a 

3 

a 
c 
< 

Time  of  Max.  temp.  (p.  m.). 

3:00 

3:30 

4:00 

3:00 

3:00 

3:00 

3:00 

3:00 

3:00 

"  Min.  rate  (p.  m.).. 

3:00 

3:00 

3:00 

3:30 

3:30 

3:(K) 

3:00 

3:(K) 

3:(M) 

3.00 

3:00 

3:1  0 

3:00 

"       "  Min.  temp.  (a.  m.). 

T:(KI 

T:00 

6:00 

6:00 

5:(J0 

5:(10 

5:00 

5:00 

6:(K) 

6:00 

7:00 

7:00 

6:00 

"       "  Min.  rate  (a.  m.).. 

7:(H) 

6;()0 

6:00 

5:30 

5:U0 

4:30 

5:00 

5:30 

6:(M) 

6:00 

6:30 

6;.30 

5:30 

"  Max.  rate  (a.  m.).. 

1000 

10:00 

10:00 

9:01) 

9:(H) 

H-.m 

S;30 

f<:(H) 

9:U0 

9:00 

9:00 

10:30 

9:30 

"       *'  Max.  rate  (p.  m.).. 

6:30 

5:30 

6:30 

7:00 

7:00 

7:00 

7;30 

7:30 

6:30    6:30 

6:30 

6:00 

6:30 

TABLE  XIII.-MEAX  HOURLY  CHANGE  OF  TEMPERATURE. 
(Expressed  in  degrees  and  tenths  of  a  degree.) 


Midn't  tol  a.  m.. 

1-  3 

2-  3 

3-  4 

4-  5 

5-  6 

6-7 

7-8 

8-  9 

9-10 

10-. 1 

11-Xoon... 
X^oon-1  p.  m.. 

1-  3 

2-  3 

3-  4...'.... 

4-  5 

5-  6. 

6-  7 

7-  « 

8-  9  

9-10 

10-n 

11-Midn't.. 


—  .4    —    .O 

—  .4    -   .4 

—  .4    —  .4 


2 •>    


.,T    —  .  t    — 


.5 

.4 
.3 
.3 

i!o 

1.7 
1.7 
1.8 
1.7 
1.3 
1.2 
.5 
.1 


1.4  - 

8, 

i 

.6 

.1 
1.0 
2.1 


1.3 

.8 


3.0 
1.9 
1.5 
1.1 


— 1. 


—1.2  -1-0  - 


—  .81  —1.1;  — : 


1.0 
1.4 

1.1; 

1.1; 
9, 


—  .3  —  .5  —  .8  —1.1  —  .6  — 


.0 
.6 
1.1 
1.7 
1.3 
1.3 
1.1 
.9, 
1.1 


-  .5 
.4 
1.6 
2.3 
2.1 
1.8i 
1.9 
1.3 
1.1 
1.0 


-1.3 
-3.0 
-1.8i 
-1.5i 
-1.1 
-1.1 
.6 


-  .8 

-  .8 

-  .6 

-  .6 
.9 

2.1 
2.3 
2.3 
1.8 
1.7 
1.3 
1.3 
.9 
.3 

-  .1 

-  .9 
-1.3 
-1.9 
-1.6 
-1.5 
-1.3 
-1.1 
-1.0 


.3 
1.8 
2.3 
3.5j 
1.9; 
1.8 
1.5 
1.0 

.6 

-  .1 
-1.0 
-1.3 
-1.7 


-  .9 

-  .6 

-  .8 

-  .6 

-  .6 
.1 

1.6 
3.6 
3.4 
2.3 
3.1 
1.2 
1.1 

!i 

-  .3 
-1.0 

-1.1 

-1.7 
-1.8 
-1.4 
-1.3 
-!.& 
-1.0 


-  .5 

-  .6 

-  .8 

-  ie 

-  .3 
.9 

2.6 
2.3 
2.5 
2.1 
1.6 
l.ll 
.8 
.3 

-  .1 
-1.3 
-1.7 
-1.8 
-1.4 
-1.6| 
-1.1 


-  .3 

-  .6 

-  .6 

-  .6 

-  .4 

-  .4 
.3 

3.0 
2.4 
2.4 
2.2 
1.7i 
l.l' 
.8 
.2 

-  .4 
-1.3 
-1.6 
-1.7 
-1.4 
-1.3 
-1.0 

•9: 


-  .5 

-  .3 

-  .4 

-  .4 

-  .4 

-  .1 
.4 

1.3 
1.5 
1.9 
1.5 
1.3 
.9 
.4 
—  .5 
-1.0 
-1.3  -1.0 
-1.1  —  .9 


-  .4 

-  .3 

-  .1 
.9 

3.0| 
1.9' 
1.9i 
1.6 
1.31 
.6 

-  .6 

-  .8 


—  .«  -1.0  -1.1  -  .8  —  .1 


—  .6 
.8,  -  .6 


-  .6 

-  .5 
.0 
.8 

1.6 
1.9 
1.9 
1.9 
1.5 
.9 
.9 


-1.3 
-1.5 
-1.2 
-1.3 

-  .9 

-  .9 

-  .7 


Table  XIII  shows  the  average  amount  of  change  in  temperature  from  hour 
to  hour  in  each  month  and  in  the  year.  The  minus  sign  preceding  a  number 
indicates  a  fall  in  temperature;  numbers  without  a  sign  show  a  rise  in 
temperature.  For  example,  from  midnight  to  one  a.  m.,  the  temperature 
falls,  on  the  average,  four-tenths  of  a  degree  in  the  month  of  January,  one 
and  four-tenths  in  April,  and  eight-tenths,  on  the  average,  for  the  entire 
year.  The  results  are  also  graphically  shown  for  each  month  in  the  year  in 
Fig.  10,  and  for  the  year,  in  Fig.  17.  The  values  are  based  on  hourly  observa- 
tions for  a  period  of  ten  years. 


In  Table  XIII,  the  average  amount  by  wliich  the  temperature  changes 
from  liour  to  hour  througliout  the  day  is  rocordeil  for  t-acli  mouth  and 
6 


76 


THE    CLIMATE    OF   BALTIMORE 


for  the  year.  These  values  are  derived  from  ten  years  of  hourly  observa- 
tions from  1893  to  the  close  of  1902,  In  Fig.  16,  the  values  are  graph- 
ically shown  for  each  month  of  the  year  in  terms  of  departures  above  and 
below  a  line  separating  the  rising  from  the  falling  temperatures.  In 
Fig.  17  the  curve  representing  the  average  hourly  rate  of  change  in 
temperature  for  the  year  is  drawn  in  connection  with  the  average  hourly 
pressure  curve.  As  noted  above  in  the  paragraph  on  the  diurnal  variation 
of  the  barometer  there  is  a  close  resemblance  between  these  two  curves, 
suggesting  some  causal  connection  between  the  diurnal  warming  and  cool- 
ing of  the  atmosphere  and  pressure  changes. 


-to 


\ 1 — ^ 

/'''T^S-s                                                                      i 

\/A    I  1  1  ^J ^     ^^_^ 

T^f-/-| rV-V ' — H h+- 

U-     jU-  -^  ^i,^±^^^ i^-^SU- 

__      _    .^V      _           2^      _±+           _    _         it        - 

:iiii::ii-±^gii:iiii:^ii#iiiii:+iiiHg 

:^::==4^:^==="=====+:^+^fci::±^";^itifc 

-  -  z^.^^^     -     -^t;^  "TV-f-^ /-^t+t 

^.^^  :::^_4__^_^_^  _^     ^^s-h-  v    -  ^' 

-^+           ^J^   J U-      -U-.^^ h^^^         J7^*^- 

==  =^=F   =i+=T=     ="  ==d==4=r--^-'f =^  4+ 

"=1^=4="=n^4r"'4'4l?r'    " 

. 1_^ i    1   1   M i M ' 1 — ' — 

— ___ 1  .  .  ^ ' -  .  .  .     ..... 1 . — . — , — _ 

+015 


Fig.   17.  — Curves  Representing    the  Average  Hourly  Pressure  (a),  and  the  Hourly 
Rate  of  Change  in  Temperature  (b),  for  the  Year. 


Mean  Daily  Temperature. 

Just  as  the  daily  change  in  altitude  of  the  sun  causes  a  daily  rise  and 
fall  in  temperature,  so  the  annual  variations  in  altitude  give. rise  to  an 
annual  rise  and  fall  in  temperature.  In  the.  diurnal  period,  the  highest 
temperature  is  attained  about  three  hours  after  the  sun  reaches  the  mer- 
idian; in  the  annual  period  the  maximum  temperature  is  reached  from 
three  to  four  weeks  after  the  sun  attains  the  greatest  elevation.  While 
the  lowest  temperature  of  the  day  occurs  about  sunrise,  the  minimum  for 
the  vear  lags  four  to  five  weeks  behind  the  time  of  lowest  seasonal  altitude 


ilAKYLAXD   WEATHER    SERVICE 


77 


of  the  Sim.  The  steady  advance  and  retreat  of  the  sun  in  his  annual 
course  would  probably  cause  a  uniform  increase  in  temperature  from  day 
to  day  from  winter  to  summer,  and  a  corresponding  decrease  to  the  winter 
months,  if  the  character  of  the  earth's  surface  were  uniform.  The  distri- 
bution of  land  and  water  surfaces  is  doubtless  responsible  for  the  irreg- 
ular character  of  the  curve  representing  the  annual  changes  of  temperature 
when  constructed  from  mean  daily  temperatures. 


TABLE  XIV.— MEAN  DAILY  TEMPERATURE. 
(Corrected  to  hourly  mean.) 


1. 

S. 
i. 
5. 
6. 

S. 

9. 
10. 
11. 
12. 
13. 
14. 
15. 
16. 
17. 
18. 
19. 
20. 
21. 
22. 
23. 
24. 
2.5. 
26. 
27. 
28. 
29. 
30. 
31. 


Jan. 

Feb. 

33.5 

33.9 

34.3 

31.5 

32.2 

33.9 

33.2 

3:3.8 

33.2 

30.9 

33.0 

32.7 

34.4 

34.3 

&3.7 

35.3 

33.0 

34.7 

32.2 

34.5 

31.  li 

.36.1 

.34.0 

36.7 

33.2 

36.1 

34.8 

a5.3 

33.1 

35.9 

34.6 

35.7 

33.8 

36.5 

33.2 

37.5 

33.9 

.37.3 

34.2 

36.3 

36.0 

37.5 

35.5 

38.1 

35.1 

37.1 

.32.2 

a5.3 

32.4 

37.4 

32.5 

37.7 

.33.8 

36.0 

.34.8 

35.9 

33.6 

36.6 

33.0 

34.3 

38.2 
38.0 
38.7 
37.8 
36.8 
39.0 
40.2 
39.5 
40.3 
43.1 
41.4 
42.8 
41.8 
40.6 
39.0 
39.9 
39.2 
39.8 
43.2 
41.4 
41.2 
42.2 
41.2 
41.0 
42.0 
42.5 
44.0 
45.0 
43.4 
44.5 
46.2 


47.0 
49.2 
48.8 
48.4 
48.7 
49.8 
49.8 
50.6 
.50.2 
50.2 
50.6 
51.2 
52.4 
.55.2 
53.2 
52.8 
52.7 
.53.4 
55.2 
55.4 
.55.9 
55.6 
57.2 
57.6 
.56.4 


.58.0 
57.4 
59.7 


59.3 
60.5 
58.9 
59.5 
61.2 
60.9 
60.7 
61.7 
64.9 
66.2 
65.8 
64.5 
62.7 
61.9 
63.8 
64.0 
63.4 
63.9 
&5.5 
66.5 
66.4 
66.1 
65.2 
65.7 
67.1 
67.5 
67.3 
68.6 
67.3 
68.8 
70.3 


rune 

July 

Aug. 

Sept. 

Oct. 

Nov. 

70.2 

75.5 

76.6 

72.4 

62.5 

51.5  ' 

69.0 

75.7 

76.4 

71.8 

61.6 

53.4 

71.2 

78.1 

76.6 

7''  7 

62.2 

50.1 

71.4 

78.7 

76.6 

72.0 

63.0 

47.7 

71.2 

77.5 

76.7 

72.4 

60.9 

48.5 

71.6 

77.. 5 

77.0 

73.6 

60.7 

49.0  , 

70.4 

77.7 

76.8 

71.8 

59.0 

48.8  ' 

72.1 

77  7 

77.0 

71.4 

.59.4 

49.6 

72.6 

77.9 

77.4 

70.6 

59.0 

49.9    ; 

72.3 

77.9 

77.5 

70.3 

59.0 

49.3 

72.6 

77.7 

76.8 

68.6 

58.7 

47.7 

72.4 

77.6 

76.8 

69.7 

.57.4 

47.1 

72.4 

78.5 

76.2 

69.6 

58.0 

46.3 

73.3 

78.2 

75.6 

67.6 

58.1 

44.9 

73.8 

78.9 

75.6 

67.1 

56.9 

45.3 

72.8 

79.6 

75.4 

69.0 

57.8 

46.2 

73.9 

78.9 

75.0 

68.8 

.57.4 

45.7 

73.8 

79.1 

75.6 

67.7 

57.2 

46.3 

74.6 

77.3 

75.4 

69.3 

56.2 

44.3 

75.6 

77.3 

75.4 

66.8 

54.7 

42.0 

75.6 

77.5 

75.8 

64.6 

54.6 

41.3 

75.2 

76.7 

75.2 

64.8 

54.3 

42.8 

75.1 

77.6 

74.3 

65.4 

55.0 

43.7  ' 

76.1 

76.8 

74.6 

65.2 

54.1 

41.7 

75.9 

7';.  3 

74.4 

64.8 

.53.1 

41.5 

77.fi 

78.7 

74.0 

65.4 

53.4 

41.6 

76.2 

78.2 

72.8 

64.0 

53.8 

41.9 

76.6 

77.1 

72.1 

64.1 

o«.o 

40.8 

76.2 

77.4 

72.6 

64.0 

.52.6 

38.1 

75.6 

77.5 

73.4 

62.9 

52.0 

36.2 

76.9 

73.2 

50.6 

36.3 
38.3 
38.5 
39.0 
37.9 
38.3 
39.3 
38.6 
38.7 
37.8 
40.0 
39.9 
39.7 
38.3 
36.9 
36.2 
36.3 
36.3 
35.9 
33.9 
a5.7 
37.3 
37.8 
36.5 
35.5 
34.3 
34.0 
33.9 
33.7 
34.1 
33.8 


Table  XIV  shows  the  mean  temperature  for  each  day  of  the  year  as  derived 
from  the  daily  maximum  and  minimum  temperatures  for  30  years,  from  1871 
to  1900.  To  the  average  daily  values  derived  from  these  observations,  cor- 
rections have  been  applied  to  reduce  them  to  the  true  mean  based  on  24  hourly 
observations.  The  altitude  of  the  thermometers  varied  from  40  to  60  feet 
above  the  ground. 


In  Table  XIV  the  average  temperature  for  each  day  of  the  year  is 
shown,  based  upon  the  daily  maximum  and  minimum  temperature  for  a 
period  of  thirty  years.     The  corrections  wliieli  were  found  necessary  in 


78  THE    CLIMATE    OF    BALTIMORE 

the  preceding  paragraphs,  in  order  to  reduce  these  values  to  the  true 
daily  mean  based  on  2 4-hour ly  observations,  have  been  applied  in  this 
table.  In  Plates  III  and  IV  the  daily  mean  temperatures  for  the  same 
period  (1871-1900)  of  thirty  years,  are  shown  graphically  in  curves  B. 
The  irregular  serrated  appearance  of  these  curves  is  very  marked.  The 
advance  and  retreat  of  the  seasons  is  accomplished  by  a  succession  of 
waves  of  rising  and  falling  temperature,  of  unequal  period,  but  averaging 
about  three  to  four  days.  These  changes  accompany  the  areas  of  high 
and  low  atmospheric  pressure  which  pass  in  continual  succession  from 
west  to  east  within  the  temperate  zones  of  the  northern  and  southern 
hemispheres,  and  which  have  become  familiar  to  us  in  the  daily  weather 
charts  now  isssued  by  nearly  all  national  governments. 

A  study  of  the  curves  representing  the  daily  temperatures  for  the  year, 
shows  a  greater  variability  in  the  winter  months  than  in  the  summer 
months.  This  is  more  readily  recognized  in  the  curves  of  extreme  tem- 
peratures (Plate  IV,  curves  A  and  C),  than  in  those  representing  the 
average  temperature  for  a  long  period  (curve  B).  In  the  past  thirty 
years  the  temperature  has  been  lowest,  on  the  average  in  Baltimore,  on 
the  5th  of  February  (30.9°).  The  day  having  the  highest  average  tem- 
perature of  the  year  is  the  16th  of  July  (79.6°).  Hence  the  temperature 
rises  during  161  days  and  falls  during  a  period  of  204  days.  From 
April  20th  to  October  23rd  the  temperature  remains  above  the  average 
for  the  year;  from  October  23rd  to  April  20th  it  is  below.  The  temper- 
ature rises  most  rapidly  in  March  and  falls  most  rapidly  in  November. 

As  stated  above,  the  temperature  of  the  air  at  the  earth's  surface  lags 
behind  the  temperature  of  direct  solar  radiation  nearly  a  month,  the  latter 
attaining  a  maximum  value  on  June  22nd,  the  former  about  July  17th, 
at  Baltimore.  This  lagging  effect  is  particularly  noticeable  in  the  tem- 
perature of  late  summer  and  the  autumn.  On  the  22nd  of  March  and  of 
September  the  direct  rays  of  the  sun  which  fall  upon  Baltimore  are 
presumably  of  approximately  equal  intensity,  as  the  sun  is  at  these  times 
directly  over  the  equator.  The  temperature  of  the  air,  however,  screened 
from  the  direct  rays  of  the  sun,  is  65°  on  September  22nd,  wliile  it  is 
only  42°  on  the  22nd  of  March.     This  marked  difference  between  the 


MARYLAXD    "WEATHER    SERVICE  79 

temperatures  of  corresponding  days  of  the  ascending  and  descending 
branches  of  the  annual  curve  holds  good  throughout  the  year. 

TABLE  XV.-MEAX  DAILY  CHANGE  OF  TEMPERATURE. 
(Expressed  in  degrees  and  tenths  of  a  degree.) 


Jan. 
—0.4 

Feb. 

Mar. 

Apr.    May  June 

July 

Aug. 

Sept. 

Oct.    Nov. 

Dec. 

0-  1 

—0.5 

1.7 

0.4    —0.5    -0.2 

0.0 

-0.2 

-0.6 

-0.2       1.0 

0.1 

1-3 

0.8 

-2.4 

-0.2 

2.2       1.2    -0.2 

0.3 

—0.2 

-0.6 

-0.9'      0.9 

2.0 

2-  3 

-3.1 

2.4 

0.7 

—0.4   —1.6       2.2 

2.4 

0.2 

0.9 

0.6    -2.3 

0.2 

^4 

1.0 

0.0 

-0.2 

-0.1 
—2.9 

1.8 

-0.9 

—1.0 

2.2 

-0.4       0.6       0.2 
0.3       1.7    -0.2 
1.1    -0.3       0.4 

0.6 
-1.2 

0.0 

0.0 
0.1 
0.3 

-0.7 
0.4 
1.2 

0.8    -2.4 

—2.1       0.8 

0.2       0.5 

0.5 

4-5 

—1.1 

5-6 

0.4 

6-7 

1.4 
-0.7 
-0.7 
-0.8 

1.6 

1.0 

-0.6 

—0.3 

1.2 

-0.7 

0.8 

2.8 

0.0   —0.2    -1.2 

0.8       1.0       1.7 

-0.4       3.2       0.5 

0.0       1.3   -0.3 

0.2 
0.0 
0.2 
0.0 

-0.2 
0.2 
0.4 
0.1 

-1.8 
-0.4 
-0.8 
-0.3 

—1.7   -0.2 
0.4       0.8 

—0.4       0.3 
0.0    -0.6 

1.0 

7    8 

— O.T 

8-  9 

0.1 

9-10 

-0.9 

10-11 

-0.6 
2.4 

1.6 
0.6 

-1.7 
1.4 

0.4   -0.4       0.3 
0.6    -1.3   -0.2 

-0.2 
-0.1 

-0.7 
0.0 

-1.7 
1.1 

-0.3   —1.6 
-1.3   -0.6 

0  •> 

11-12 

-0.1 

12-13 

-0.8 

-0.6 

-1.0 

1.2    -1.8       0.0 

0.9 

-0.6 

-0.1 

0.6   -0.8 

-0.2 

13-14 

1.6 

-0.8 

—1.2 

3.8   -0.8       0.9 

—0.3 

-0.6 

—2.0 

0.1    -1.4 

—1.4 

14-15 

-1.7 

0.6 

-1.6 

—2.0       1.9       0.5 

0.7 

0.0 

-0.5 

-1.2       0.4 

-1.4 

15-16 

1.5 

-0.2 

0.9 

-0.4       0.2   —1.0 

0.7 

-0.3 

1.8 

0.9       0.9 

-0.7 

16-17 

-0.8 

0.8 

—0.7 

—0.1       0.6       1.1 

-0.7 

-0.4 

-<l.2 

-0.4    -0.5 

0.1 

17-18  

-0.6 

0.1 

0.6 

0.7       0.5    -0.1 

0.2 

0.6 

-1.1 

-0.2       0.6 

0.0 

18-19 

0.7 

-0.2 

3.4 

1.8       1.6       0.8 

-1.8 

-0.2; 

1.6 

—1.0   -2.0 

-0.4 

19-20 

0.3 

-0.1 

—1.8 

0.2       1.0       1.0 

0.0 

0.0 

—2.5 

—1.5    -2.3 

-2.0 

20-21 

1.8 

1.2 

-0.2 

0.5    —0.1       0.0 

0.2 

0.4 

—3.2 

-0.1    -0.7 

1.8 

21-22 

—0.5 

0.6 

1.0 

-0.3    -0.3    -0.4 

-0.8 

-0.6 

0.2 

-0.3       1.5 

1.6 

22-23 

-0.4 
—2.9 

-1.0 
-1.8 

-1.0 
-0.2 

1.6   —0.9    -0.1 
0.4       0.5       1.0 

0.9 

-0.8 

—0.9 
0.3 

0.6 
-0.3 

0.7       0.9 
—0.9   —2.0 

0.5 

23-24 

-1.3 

24-25 

0.2 
0.1 

2.1 
0.3 

1.0 
0.5 

—1.2       1.4    -0.2 
1.1       0.4       1.7 

0.5 
1.4 

-0.2 
-0.4 

-0.4 
0.6 

-1.0   -0.2 
0.3       0.1 

-1.0 

2.5-26 

-1.2 

26-27 

1.3 

-1.7 

1.5 

0.3    -0.2    —1.4 

-0.5 

-1.2 

-1.4 

0.4       0.3 

-0.3 

27-28 

1.0 

-0.1 

1.0 

0.2       1.3       0.4 

—1.1 

-0.7; 

0.1 

-1.5    -1.1 

-1.1 

28-29 

-1.3 

0.7 

-1.6 

—0.6    —1.3   —0.4 

0.3 

0.5 

-0.1 

0.3   -2.7 

0.8 

29-30 

-0.6 

1.1 

2.3       1.5    -0.6 

0.1 

0.8 

-1.1 

-0.6   -1.9 

0.4 

30-31 

1.3 
1.0 

1.0 

1.7 
1.2 

1.5 

-0.6 

-0.2 

-1.4 

-0.3 

0.8       1.0       0.6 

0.6 

0.4 

0.9 

0.7       1.0 

0.8 

Table  XV  shows  the  change  in  the  mean  daily  temperature  from  day  to 
day  throughout  the  year.  The  minus  sign  indicates  a  fall  in  temperature 
while  absence  of  the  sign  indicates  a  rise.  For  example:  The  1st  of  January 
is  0.4°  colder,  on  the  average,  than  the  day  preceding;  the  9th  of  May  is  3.2" 
warmer  than  the  8th  of  May,  etc.  The  same  results  are  shown  in  curves  B 
of  Plates  III  and  IV.  These  values  are  based  on  daily  average  temperatures 
for  a  period  of  thirty  years. 


Average  Inter-Diurnal  Changes  of  Temperature. 

The  changes  in  the  average  temperature  from  day  to  day  are  indicated 
with  greater  accuracy  in  Table  XY  than  in  the  curves  on  Plates  III  and 
IV.  The  amount  of  rise  or  fall  in  temperature  from  day  to  day  through- 
out the  vear  is  given  to  tenths  of  a  degree.     The  average  variability  is 


80 


THE    CLIilATE    OF    BALTi:kIORE 


approximately  0.8°,  and  varies  from  1.0°  or  1.2°  in  the  winter  months 
to  0.5°  or  0.6°  in  the  summer  months  when  the  changes  are  considered 
without  reference  to  sign.  The  month  of  greatest  variability  is  March, 
while  July  is  the  month  of  least  variability.  The  annual  march  of  tem- 
perature shows  some  interesting  periods  of  marked  rise  and  fall,  per- 
iods of  three  or  more  days  during  which  there  is  a  conspicuous  departure 
from  the  path  representing  a  steady  progressive  change.     Such  periods 


YiG.   18. — Inter-diurnal  Temperature  Changes. 

Fig.  18  sliows  tlie  average  monthly  frequency  of  changes  of  stated  amounts  in  the  mean 
temperature  of  the  day  from  day  to  day.  The  marginal  figures  indicate  the  degree  of 
change,  and  the  heavy  curved  lines  the  frequency  of  stated  changes.  For  example,  a  change 
of  2°  in  the  mean  daily  temperature  occurs  on  the  average  3.2  times  in  .January,  2.6  times  in 
February,  4.7  times  in  June,  and  5.-5  times  in  August,  etc.  Increase  in  the  intensity  of  the 
shading  represents  increase  in  the  frequency  of  occurrence.    See  also  Table  XVII, 

have  received  a  great  deal  of  attention  from  European  climatologists  and 
there  is  an  abundant  literature  of  a  popular  as  well  as  scientific  character 
grouped  about  these  special  days.  Throughout  central  and  southern  Eu- 
rope there  is  a  popular  impression  that  iiijuriou.s  frosts  are  likely  to  occur 


MARYLAND  WEATHER  SERVICE. 


VOLUME  2,  PLATE  IN. 


5   IOI520  2530  5   lOlsaoaS      5    10  l-j  gO  SS-^U  ,  ..i^^^  30  ^  iOi.,,.,,.3::.:=m«mnim^^  |         " '%,7„!^ 


A.     Average  daily  maximum  temperature.  B.     Daily  mean  temperature.  *'^^^ ''''''^  "unimuni  temperature  D      D 


aily  mean  barometric  pressure. 


MARYLAXD   WEATHER    SERVICE  81 

in  the  early  part  of  May,  and  their  coming  is  awaited  with  anxiety  by 
the  agricultural  classes,  especially  in  France  and  northern  Germany. 
May  11,  12  and  13,  are  the  days  of  most  probable  occurrence  and  these 
days  have  been  variously  designated  as  the  "  Three  Ice  Saints,"  the  "  Three 
Ice  Men,"  etc.,  by  the  husbandmen.  Similar  regressions  in  temperature 
have  been  noted  at  other  seasons  of  the  year  and  carefully  investigated 
but  the  spring  drop  occurring  at  a  critical  period  in  crop  growth  has 
received  by  far  the  largest  share  of  attention. 

In  a  study  of  the  tendency  to  the  formation  of  frosts  from  the  10th  to 
the  13th  of  May  in  Europe,  Dr.  Assmann  has  shown  that  the  fall  in 
temperature  is  first  shown  in  Scandinavia,  spreads  in  a  southerly  and 
then  in  a  southwest  direction  over  Central  Europe.  The  maximum  de- 
parture is  attained  on  the  10th.  Eeceding  eastward,  at  first  slowly  and 
then  more  rapidly,  it  reaches  the  Russian  provinces  of  the  Baltic  on  the 
13th.  Daily  weather  charts  constructed  by  van  Bebber  show  a  pro- 
gressive departure  of  pressure  which  readily  accounts  for  a  period  of 
northerly  winds  and  clear  skies  and  abnormally  low  temperature,  first 
over  northern  Europe,  then  southward  over  central  Europe  and  France. 

Careful  study  of  the  daily  temperature  at  Baltimore  does  not  dis- 
close a  tendency  toward  a  decided  fall  in  temperature  on  these  days.  On 
the  contrary,  there  is  a  distinct  rise  from  the  9th  to  the  12th  in  place  of 
the  European  fall  (see  curves  A.  B.  and  C,  Plate  III).  A  similar  plus 
departure  in  temperatures  at  this  time  is  disclosed  in  the  daily  temper- 
ature curves  of  other  localities,  namely,  Washington,  Norfolk,  Nashville, 
Columbus,  Ohio.  The  geographical  extent  of  this  marked  departure  has 
not  yet  been  carefully  investigated,  but  it  is  not  confined  to  the  localities 
named.  A  probable  explanation  of  this  phenomenon  may  be  found  in  a 
periodic  recurrence  at  this  time  of  an  area  of  high  barometric  pressure 
over  the  South  Atlantic  states,  or  an  extension  westward  of  the  permanent 
area  of  high  pressure  over  the  North  Atlantic  in  latitude  of  about  30° 
in  conjunction  with  the  development  of  a  barometric  depression  in  the 
Mississippi  Valley.  Such  a  pressure  distribution  invariably  causes  a  rise 
in  temperature  above  the  average  for  the  time  of  year  in  the  central 
states  and  the  Middle  Atlantic  states. 


82 


THE    CLIMATE    OF    BALTIMOUE 


A  similar  period  of  marked  rise  occurs  at  Baltimore  about  March  7  to 
13  and  again  April  12  and  13.  About  June  20,  and  again  about  July  20, 
there  is  apparently  a  tendency  for  the  temperature  to  fall  below  the 
normal  for  two  or  three  days.  These  marked  departures  from  the  steady 
and  uniform  seasonal  advance  or  retreat  of  temperature  conditions  do 
not  always  occur  upon  the  same  days,  year  after  year,  but  the  fact  that 


TABLE  XVI.— MEAN  DAILY  RANGE  OF  TEMPERATURE. 


Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

1 

12.0 

11.3 

14.4 

14.7 

18.0 

19.0 

15.9 

16.0 

15.9 

17.8 

15.9 

13.3 

2 

15.0 

13.9 

13.3 

17.1 

18.4 

17.1 

16.1 

15.3 

16.9 

16.7 

17.8 

14.1 

3 

13. 5 

14.5 

14.4 

17.2 

17.5 

16.6 

18.9 

15.9 

18.6 

18.2 

14.8 

13.7 

4 

13.1 

13.2 

14.5 

15.4 

16.9 

16.1 

17.9 

16.3 

17.1 

15.9 

15.3 

12.6 

5 

13.9 

12.5 

14.9 

17.4 

18.0 

17.4 

15.7 

16.6 

17.7 

16.4 

15.5 

13.4 

6 

11.7 

14.4 

15.9 

16.5 

19.1 

16.9 

16.4 

16.9 

16.9 

16.0 

15.2 

13.8 

12.0 

13.9 

16.5 

16.7 

16.3 

16.0 

17.5 

16.2 

16.5 

12.9 

16.0 

15.0 

8 

13.6 

13.6 

13.6 

16.5 

16.3 

17.4 

17.5 

17.3 

14.7 

16.5 

14.6 

13.3 

9 

13.8 

12.7 

14.4 

16.1 

21.4 

17.9 

10.1 

17.3 

16.1 

17.6 

14.7 

14.1 

10 

14.3 

14.5 

14.8 

15.2 

18.8 

16.7 

17.3 

15.x 

16.6 

19.1 

13.6 

14.2 

11 

12.7 

13.3 

15.8 

16.3 

18.0 

18.1 

17.5 

17.7 

13.2 

18.4 

12.7 

13.4 

12 

13.0 

13.5 

15.5 

17.5 

18.3 

17.7 

17.6 

16.3 

14.6 

14.3 

15.1 

13.3 

13 

12.4 

12.5 

15.6 

17.9 

18.3 

17.6 

17.3 

15.9 

14.6 

14.9 

16.3 

14.9 

14 

14.1 

13.1 

13.6 

18.2 

15.5 

18.3 

17.4 

15.5 

13.7 

14.6 

14.0 

14.4 

15 

12.4 

15.fi 

14.2 

15.2 

17.8 

17.2 

17.5 

15.6 

14.8 

18.1 

13.9 

14.2 

16 

12.6 

15.6 

13.8 

16.1 

17.4 

16.7 

18.0 

16.2 

16.8 

19.1 

14.8 

14.0 

17 

13.7 

15.9 

14.1 

15.8 

17.8 

16.2 

16.9 

15.7 

17.7 

18.5 

14.4 

13.9 

18 

12.9 

15.4 

15.7 

16.0 

17.0 

17.3 

16.7 

16.2 

16.6 

17.2 

14.9 

14.4 

19 

11.2 
13.4 

14.1 
14.6 

14.9 
13.6 

17.9 
16.3 

17.3 
16.2 

18.7 
19.2 

15.5 
16.4 

16.6 
17.3 

16.6 
15.4 

17.3 
16.6 

13.5 
11.8 

13.0 

20 

12.3 

21 

12.7 

15.1 

15.7 

18.2 

17.2 

17.3 

15.8 

17.3 

15.9 

16.7 

14.3 

13.6 

22 

14.4 

13.7 

15.3 

17.5 

16.9 

17.3 

16.6 

16.1 

17.2 

16.2 

13.4 

13.9 

23 

15.2 

14.9 

19.9 

18.5 

15.8 

17.8 

16.2 

14.8 

16.1 

15.1 

13.5 

14.6 

24 

12.3 

15.1 

17.3 

17.5 

16.9 

17.8 

16.0 

15.3 

15.2 

14.6 

12.8 

13.1 

25 

13.5 

15.3 

15.5 

15. 9 

16.5 

17.4 

16.7 

16.7 

16.3 

14.6 

12.4 

12.0 

26 

14.8 

14.2 

14.6 

17.4 

15.9 

17.0 

16.7 

16.2 

16.6 

15.9 

13.6 

11.6 

27 

14.4 
12.2 

14.8 
14.7 

14.1 
13.5 

17.3 
17.5 

16.3 

18.8 

15.3 
17.1 

15.0 
15.7 

14.5 
15.4 

16.1 
16.8 

14.2 
13.0 

15.1 
12.2 

12.6 

28 

12.1 

29 

14.5 

9.6 

14.5 

15.7 

17.9 

16.7 

16.2 

15.9 

15.5 

14.3 

13.3 

13.9 

30 

13.5 

16.8 

20.2 

18.4 

10.3 

15.4 

15.9 

15.6 

13.7 

11.4 

12.4 

31 

11.8 

15.9 

16.7 

15.5 

16.3 

14.7 

13.6 

Table  XVI  shows  the  average  difference  between  the  daily  maximum  and 
daily  minimum  temperatures  for  each  day  of  the  year,  based  on  observations 
covering  a  period  of  30  years. 


they  persist  in  a  curve  representing  average  conditions  for  a  long  series 
of  years  (over  thirty  years  in  the  Baltimore  series)  would  seem  to  point 
to  a  decided  tendency  toward  the  formation  of  a  given  system  of  pressure 
distribution  upon  these  days.  This  subject  of  the  periodic  recurrence  of 
similar  weather  types  is  a  matter  worthy  of  more  attention  than  has  yet 
been  given  to  it  in  this  country. 


MARYLAND  WEATHER  SERVICE, 


VOLUME  2,  PLATE  I 


5   10  I5?02530   5   10  15  2025      i   lO   15  202530   5   10  I5202S30  5   I 


BASED    UPONDAiaO-^-^^'^NSPOU  30,,,^^^ 


A.     Daily 


B      n-,  ,  C      Daily  minin'""?"^''^^-         q.     Ev 

temperature.  B.     Daily  mean  temperature.  >-.     ^^  l.\ 


treme  range  of  temperature.  E.     Average  daily  range  of  temperature. 


MARYLAND    WEATHER    SERVICE 


83 


Average  Daily  Kaxge. 

The  average  maximum  and  minimum  temperatures  for  each  day  of  the 
3'ear  for  a  period  of  thirty  years  are  shown  graphically  in  curves  A  and  C 
on  Plate  III.  These  curves  show  the  characteristics  already  described  in 
considering  the  mea7i  daily  temperatures,  which  was  to  be  expected  as  the 
latter  were  derived  from  the  daily  maximum  and  minimum.  The  average 
daily  range  of  temperature,  or  the  average  difference  between  the  highest 
and  lowest  readings  for  each  day,  is  shown  in  Table  XVI.  The  daily 
range  is  also  shown  on  Plate  III  by  the  difference  in  value  of  correspond- 
ing points  in  curves  A  and  C  and  directly  in  curve  E  on  Plate  IV.  Dur- 
ing the  winter  months  the  range  is  least;  it  increases  in  the  spring 
months,  reaching  a  maximum  in  May  and  June,  then  decreases  steadily 
to  a  minimum  in  January.  As  the  daily  range  is  largely  dependent  upon 
the  amount  of  cloudiness  and  atmospheric  movement,  as  shown  in  pre- 
ceding paragraphs,  a  considerable  variation  in  the  range  from  year  to 
year  in  the  same  month  may  be  expected.  In  January,  for  instance,  with 
a  range  of  13.3°  as  an  average  for  thirty-two  years,  it  has  varied  from 
11.2°  in  1891  to  17.0°  in  1876.  The  March  range  has  varied  from  11.8° 
in  1891  to  22.0°  in  1873.  The  smallest  average  range  for  any  month 
occurred  in  Xovember  1884,  namely  10°  ;  the  greatest  average  was  that  of 
March,  1873,  with  22.0°.  When  the  daily  range  is  averaged  up  for  an 
entire  year  the  variability  is  reduced  to  comparatively  narrow  limits. 
The  annual  average  was  smallest  in  the  year  1882  (14.1°)  and  greatest 
in  1900  (16.9°).  The  ten-year  averages  have  varied  only  between  the 
limits  15.2°  and  15.9°. 


AVERAGE  DAILY  RANGE  OF  TEMPERATURE. 


Period.  Jan.    Feb.   Mar.  Apr.    May   June  July   Aug.  Sept.  Oct.    Nov.  Dec.  Year 


1871-1880 13.8  14.2  15.9 

1881-1890 13.5  14.4   14.6 

18!tl-HK)0 13.5  13.8  15.0 


1871-1902 13.2 


14.1 


15.3 


16.1 
17.1 
17.2 

16.7 


17.6 
16.6 
18.0 

17.4 


16.7 
17.0 
18.0 

17.4 


16.3  14.8  15.7 

16.2  15.7  ;  15.1 

17.3  17.3  I  17.3 


16.7 


16.0  16.1 


16.8 
16.0 
16.7 


14.0  13.6 

13.5  13.3 

14.6  13.4 


16.3  14.0 


13.4 


15.5 
15.2 
15.9 


Diurnal  Variability  of  Temperature. 
A  climatic  factor  of  the  highest  importance,  especially  to  those  who  are 
not  in  the  best  of  healtli,  is  the  variability  of  tomporature  conditions  from 


84 


THE  CLIMATE  OF  BALTIMORE 


day  to  day.  The  magnitude  of  the  diurnal  change  may  be  represented  in 
various  ways :  either  by  comparing  the  extremes  of  temperature  of  one  day 
with  those  of  the  following,  or  the  average  daily  temperatures,  or  readings 


o' 

)° 

2° 

3° 

\° 

5' 

6' 

7" 

B" 

9° 

10 

1  2 

14^ 

16 

18° 

20" 

2 

2 

" 

24 

• 

2 

' 

; 

1 

I 

, 

I 

,     1 

; 

1 

! 

- 

"V 

- 

1 

1      j 

- 

/ 

l_j  ! 

' 

\ 

1 

/; 

f 

/  1 

-4^ 

- 

V 

1  i 

\ 

/ ' 

' 

I 

1 

\\ 

\ 

|. 

\ 

1 

1 

V 

1 

> 

, 

I 

Year 

, 

\ 

1 

> 

: 

V 

I 

\ 

I 

V 

> 

^ 

- 

- 

1 

\ 

■ 

1    1 

V 

\ 

^ 

\ 

j 

V 

1 

y- 

i 

\ 

^ 

\ 

\\ 

/ 

s 

\ 

/l 

^ 

k 

N. 

/ 

.-  ^ 

1  * 

. 

^    y 

\ 

V 

b 

"■•*•.    s 

V 

\ 

1 

'''<  ]  ' 

**. 

L. 

\ 

c 

/  ^ 

' 

^ 

■^T^ 

s 

d 

J 

/ 

V 

V 

s 

/ 

s 

'^ 

^ 

e 

s 

^ 

■^ 

s^ 

v 

'^ 

V 

s 

k 

V 

>s 

'v 

-^ 

_   \ 

*>^ 

- 

— 

_ 

_ 

_ 

_ 

_ 

I- 

_ 

_ 

^ 

-- 

^.~ 

-iaT?^ 

=:) 

.:^ 

S:?-^ 

= 

1 1  - 



J 

_ 

_ 

_ 

_ 

_ 

~  1 

- 

n 

— 

- 

- 

- 

- 

- 

-n- 

H 

^ 

f 

'-^ 

■^ 

" 

r^^ 

T 

" 

" 

-1 

Fig.  19.— Total  Seasonal  and  Annual  Frequencj'  of  Stated  Diurnal  Chang-es  of 
Temperature. 

(a)  Total  Annual  frequency.  [d)  Total  Spring  frequency. 

(b)  "      Summer         "  (e)       "      Winter         " 

(c)  "      Autumn         " 

rig.  19  shows  the  total  number  of  stated  changes  In  the  mean  daily  temperature  during 
each  season  and  during  the  year.  The  upper  horizontal  line  of  figures  indicates  the  degree 
of  change,  and  the  marginal  figures  to  the  right  of  the  diagram  show  the  frequency  of 
stated  changes.    See  also  Table  XVII. 


made  at  the  same  hour  of  the  day.  The  frequencj  of  changes  of  a  given 
amount  will  depend  somewhat  upon  the  method  chosen  for  determining 
the  daily  change.     "We  have  seen  above  that  the  normal  change  in  temper- 


MARYLAND    WEATHER    SERVICE 


85 


ature  from  day  to  day  varies  from  0.5°  or  0.6°  in  the  summer  months  to 
1.0°  or  1.2°  in  the  winter  months.  But  this  average  change  for  a  long 
series  of  years  is  of  less  significance  than  the  frequency  of  changes  of  a 
given  amount.  Large,  and  especially  sudden,  changes  in  temperature 
within  short  periods  have  never  been  considered  particularly  desirable 
from  any  point  of  view.  Such  changes  may  have  advantages,  but  the  un- 
comfortable, if  not  actually  harmful  effects,  of  rapid  changes  are  sure  to 
outweigh  these.  As  a  general  rule  proximity  to  the  ocean  will  insure  an 
equable  temperature,  free  from  sudden  and  large  changes.     Especially 


Jan. 

Feb. 

MCH.          Apr.           May 

UNE 

July 

Aug. 

Sept 

Oct 

Mo 

/. 

De 

-^-^^ 

i 

1 

\ 

_ 

1 
-| — \— 

r+4- 

1 

1 

I    1 

- 

_^_ 

-i — 

- 

"IP 

?= 

-■ 

■= 

-- 

^s 

-*- 

r 

- 

-j- 

-^ 

^\ 

_L 

t^ 

1 

k 

N 

i 

1      1 

!  ] 

^■*^ 

"v. 

1 

'  1 1 

1 

. 

1  1 

1 

^ 

^ 

1 

k 

,, 

-6 

1 

"SJ" 

Jf^ 

^^~ 

r^i 

.  i  1 

r 

^ 

~" 

T 

1  1  1 

-, 

^ 

^""^^ 

;    , 

^N 

j 

^ 

-4- 

rf' 

^-i 

""i 

P 

-U 

r 

-- 

— 

— 

H 

J^ 

^< 

^- 

k_. 

--4-J 

r 

-- 

-- 

-- 

- 

-,^- 

-- 

-- 

1 

1     1 

1 

"^^ 

y 

^ 

1 

t 

s 

/ 

k.* ' 

-I-l 

4- 

==m:;- 

^^; 

f- 

h- 

t^ 

^  = 

_L 



^ 

^'_ 

=  = 

c 

►_. 

-t^ 

1    1 

— +^ 

44^-"«' 

1      "" 

^-^ 

■^ 

- 

~: 

-- 

n 

T 

s* 

-j — 

-i. 

w^ 

^ 

V'- 

1    1 

..II 

1     ;     , 

^^^. 

iit^ 

1 

-I 

-- 

L_ 

-HI 

1     1     '     i 

: 

! 

'     '     ! 

1   i   1 

' '  I ' 

Fig.  20.— Diurnal  Changes  of  Temperature  of  less  than  6°,  6°  +  ,  S°+  and  10°  + 
each  month. 

Fitr.  30  shows  the  f reaucncy  of  changes  of  0°  to  5°,  of  6°  and  above,  8°  and  above,  and  of  10° 
and  above,  in  the  mean  temperature  of  the  day  for  each  month  of  the  year.  The  degree  of 
change  is  indicated  by  the  curved  lines  marked  —6°,  6°+,  8°+,  and  10°+  respectively, 
while  the  frequency  of  the  stated  changes  is  shown  by  the  marginal  figures  to  the  right  of 
the  diagram.  For  example,  a  rise  or  fall  of  .5°  or  less  in  the  mean  temperature  of  the  day 
occurs  on  the  average  1"  times  in  January,  21  times  in  May,  and  25  times  in  July,  etc. ;  a  rise 
or  fall  of  l(J°or  more  in  the  mean  daily  tempemture  occurs  6  times  in  January,  2  times  in 
May,  etc.    See  Table  XVII. 

is  this  true  on  small  islands,  or  along  the  western  coasts  of  the  continents 
where  ocean  winds  prevail.  The  diurnal  variability  increases  rapidly  as 
the  interior  portions  of  the  continental  areas  are  approached. 

The  changes  in  the  daily  average  temperature  at  Baltimore,  covering 
a  period  of  thirty  years,  have  been  computed  and  arranged  according  to 
fro<liioncy  and  degree  of  change.     These  diurnal  changes  have  also  been 


86 


THE    CLIMATE    OF   BALTIMORE 


grouped  according  to  months,  seasons,  and  the  jesn,  in  the  table  which 
follows  and  presented  graphically  in  Figs.  18,  19,  20  and  21.  In  the 
summer  months  changes  of  one,  two,  and  three  degrees  largely  predomi- 
nate, from  which  there  is  a  rapid  decrease  in  frequency  of  larger  changes. 
In  the  winter  months  changes  of  one,  two,  and  three  degrees  are  still 
dominant,  but  the  decrease  in  frequency  as  the  changes  increase  is  much 
more  gradual. 

TABLE  XVII.-FREQUEXCY  OF  STATED  DIURNAL  CHANGES  OF  TEMPERATURE. 


£ 

0) 

< 

>> 

o 

3 

3 
1-5 

3 
< 

*3 

C 
® 

CO 

O 

> 

o 
2; 

c5 

ii 
a 

Summer. 
Autumn. 

1 

0° 

2.0 

1.9 

1.7 

3.5 

2.5 

2.9 

3.5 

3.7 

3.6 

1.7 

1.9 

l.S 

6.710.2   8.3 

5.7 

.30.8 

1 

3.0 

3.4 

3.1 

3.2 

3.9 

4.4 

5.5 

5.4 

4.4 

3.6 

3.1 

3.1 

10.215.311.1 

8.545.0 

2 

3.2 

3.6 

3.6 

3.4 

4.3 

4.7 

5.6 

5.5 

4.3 

3.8 

3.6 

3.4 

11.115.711.7 

9.347.8 

3 

3.3 

3.7 

3.3 

3.4 

3.9 

4.2 

6.0 

4.9 

4.2 

4.2 

3.9 

3.5 

10. 6,14.012.3 

9.446.3 

4 

3.7 

3.7 

2.6 

3.3 

3.3 

3.3 

3.3 

3.3 

2.9 

3.5 

3.6 

3.0 

9.1   9.610.0 

8.437.1 

5 

3.6 

3.7 

2.6 

3.3 

3.0 

3.7 

3.6 

2.5 

3.6 

3.2 

3.3 

3.0 

8.9'  7.8  9.0 

8..3'.34.0 

6 

3.3 

3.3 

2.2 

3.6 

3.3 

3.0 

1.8 

1.5 

1.9 

3.3 

3.4 

2.5 

7.0  5.3  6.5 

6.925.7 

7 

2.1 

2.0 

2.0 

2  2 

1.9 

1.7 

1.3 

1.1 

1.8 

2.1 

3.0 

2.1 

6.1   4.2  5.8 

6.223.2 

8 

1.8 

1.7 

1.6 

lA 

1.4 

1.2 

0.7 

0.7 

1.2 

1.5 

1.4 

1.5 

4.5  2.6  4.2 

5.016.3 

9 

1.7 

1.5 

1.8 

1.3 

1.3 

0.9 

0.5 

0.5 

1.0 

1.3 

1.3 

1.3 

4.1  1.91  3.5 

4.614.1 

10 

1.3 

1.1 

1.6 

0.8 

0.9 

0.5 

0.3 

0.3 

0.6 

0.7 

0.9 

1.0 

3.3  1.1  2.5 

3.510.2 

11 

1.1 

0.9 

1.3 

0.7 

0.7 

0.3 

0.3 

0.1 

0.6 

0.6 

0.8 

1.0 

2.6  0.7  2.0 

3.0  8  3 

13 

1.0 

0.6 

0.7 

0.5 

0.4 

0.2 

0.1 

0.1 

0.3 

0.4 

0.6 

0.7 

1.7  0.4   1.3 

2.3;  5.6 

13 

0.8 

0.6 

0.6 

0.5 

0.3 

0.1 

0.1 

0.1 

0.3 

0.3 

0.4 

0.6 

1.3  0.3  1.0 

2.01  4.7 

14 

0.6 

0.4 

0.5 

0.3 

0.1 

0.1 

0.1 

0.3 

0.3 

0.5 

l.Oj  0.1;  0.5 

1.5j  3.0 

15 

0.3 

0.5 

0.4 

0.2 

0.1 

0.1 

0.1 

0.1 

0.2 

0.3 

0.4 

0.7  0.2'  0.5 

1.2  2.6 

16 

0.3 

0.3 

0.3 

0.1 

0.1 

0.1 

0.1 

0.3 

0.4  0.1  0.3 

0.8  1.6 

17 

0.1 

0.3 

0.2 

0.1 

0.1 

0.1 

0.2 

0.3  ..     0.3 

0.6   1.3 

18 

0.1 

0.2 

0.1 

0.1 

0.1 

0.1 

0.2 

0.2;  .. 

0.2 

0.5i  0.8 

19 

0.1 

0.1 

0.1 

0.1 

0.1 

0.1 

0.1 

0.2 

0.3 

0.4  0.8 

20 

0.1 

0.1 

0.1 

0.1 

0.1 

0.1 

0.1 

0.1 

0.3  0.6 

21 

0.1 

0.1 

0.1 

0.2  0.3 

29 

0.1 

0.1   0.2 

^ 

0.1 

0.1  0.2 

24 

0.1 

oil 

0.1  0.2 

25 

0.1 

0.1  0.1 

26 

0.1  0.1 

The  first  column  indicates  the  degree  of  change  from  day  to  day  in  the 
average  daily  temperature.  The  figures  in  the  remaining  columns  show 
the  average  monthly,  seasonal  and  annual  frequency  of  occurrence  of  indi- 
cated changes,  based  upon  observations  during  30  years,  from  1871  to  1900. 


The  larger  daily  changes  decrease  in  frequency  on  the  approach  of  the 
summer  month.*.  A  cliange  of  10°  in  the  average  temperature  of  two 
consecutive  days  has  occurred  about  3.5  times  in  30  years  during  each 
winter  month  and  9  times  in  each  of  the  months  of  Julv  and  August. 


MARYLAND  WEATHER  SERVICE 


VOLUME  2,  PLATE  I 


A.     Daily  maximum  temperature. 


B.     Daily  mean  temperature. 


BASED    UPON    D.«lV0'^^^'ONSPo„3„^.^ 

C.     Daily  ."ini"'""*"""^'        D.    £ 


reme  range  of  temperature.  E.     Average  daily  range  of  temperature. 


MARYLAND    WEATHER    SERVICE 


87 


The  details  of  these  changes  are  shown  in  the  table  above  and  in  Figs. 
18,  19,  20  and  21. 

These  figures  and  the  diagrams  reveal  the  interesting  fact  that  the 
average  departure  and  the  most  frequent  departure  are  not  identical,  or 
that  the  arithmetical  mean  of  all  departures  for  any  given  month  is  not 
the  most  probable  value.  In  the  winter  months  the  average  departure  is 
about  1°,  in  the  summer  months  it  is  about  0.6°.  The  most  probable 
departure  is  in  all  months  larger  than  the  average  departure,  as  is  clearly 
brought  out  in  the  following  comparison  of  the  average  change  from  day  to 
day  and  the  most  probable  change. 

DIURNAL  VARIABILITY. 


Jan. 

Feb. 

Mar. 

Apr. 

May  June 

Julj' 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Year 

Average  change.  .± 
Most    probable 
change ± 

1.0° 
3° 

1.0° 
4° 

1.2° 
2° 

0.8" 
3^ 

1.0° 
2° 

0.6° 

2° 

0.6° 

2° 

0.4° 

0.9° 
1° 

0.7° 
3° 

1.0° 
3° 

0.8° 
3° 

0.8° 
2° 

The  true  measure  of  diurnal  variability  is  the  change  in  the  average 
temperature  from  day  to  day.  It  is  more  convenient,  however,  to  express 
this  cliange  by  means  of  the  difference  between  the  highest  or  the  lowest 
temperature  of  successive  days,  or  between  the  temperature  at  any  given 
hour,  as  8  a.  m.  of  one  day  to  8  a.  m.  of  the  following  day,  or  from  8  p.  m. 
to  8  p.  m.  The  results  will  differ  somewhat  according  to  the  method 
adopted.  Assuming  as  correct  the  variability  as  measured  by  means  of 
the  daily  average  temperature,  the  variability  based  on  differences  of  the 
minimum  temperature  from  day  to  day  will  give  too  many  changes  under 
()°,  while  those  based  on  the  8  a.  m.,  8  p.  ni.  and  the  maximum  tempera- 
tures yield  too  few  small  changes,  the  departure  from  the  true  frequency 
being  in  the  order  named.  On  the  other  hand  when  we  consider  the  larger 
changes  of  G°,  8°,  or  10°  and  above,  the  minimum  temperature  yields  too 
many.  Take  as  an  illustration  the  frequency  of  changes  of  10°  and  over. 
During  the  course  of  a  year  of  average  temperature  conditions  there  are  -11 
diurnal  changes  of  10°  or  more,  basing  the  count  on  the  daily  minimum, 
59  on  the  8  a.  m.,  58  on  tlie  8  p.  m.,  and  83  on  the  maximum  temper- 
atures. That  is  to  say,  the  variability  computed  from  differences  in  the 
uiaxiimiiii  temperature  from  day  to  day  may  show  more  than  double  tlie 


MARYLAND  WEATHER   SERVICE. 


jtN     Apr.   July    Oct      Jan.   Apb,    Jul 


1817  1818 

Oct.     Jan     Apr.    July    Oct     Jan      Afr.  Jl 


:         .                       [  ,              [           ^                 -| 

ill.''           1                  '                      ;    i       I    '           1 

'              '                          1 

1    I        1    '        '    .            I                   1                       ;    '        I    ■  /  L  ' 

'   '      iv'M'            'aN.                 i        A 

-^ — ^^-    '  '  -    ~^'  '"  "t 1  ■    1  y  V 

1  yj^    i    ■  J  *"  "^   /a  fc/  \     /^  /^  y^L- 

f~                            '     '  /    '■^ 

^"^— ^ ^     \ '  A\  -•     \  f  \i   \y^     i\/  \ 

'      S/ 1 

1    y         r  j!   ' T 

1                                                                                        1    '" 

iL  i  -I-    "■    """"               -U- 

_u-'|-           -(---   p.-    ..      -                 ! 

1  1  1  1  1             i  !     M     i  1        1'            ' 

1820T:                            1821                       X        1822^     IL 

1   1823  i                  ;    i       1   1824  1       !                      II 

1"  ■  r           1  'J  ■■■'  T   j  1-  - 

1  1    [|         Mill!,.!           i  1 

^  1        ;    j            !                    ■    1        1    L       i  4            1        ■          A  ' 

!     1                                                        ■     I          '            \                 ■■                    'A 

/\              I      1                   1               _l            A                  '      »          /V           i      ■                 /^  ''+\ 

__^^    /\.  ^\       y\i       i          ij\                    11       i/\ 

/  ^^  I  1     '       Jhii             1        f^             /V    /    ^          I        _/         '    *' 

1                      '        1           I       /          ^            •^''~~>*                  J        ^"-^   Xi               Ki-       ,1   —^ 

/"  !    \  /^   lA     ^  /  V    /  ^ '  \  ,         \  /\  .     !  i 

\    \       1          Y/,          ^^^     ,   ^  — ..^  /                ^~T^     ^r 

'        V '         '    -^    \f  1  P '''^                           1 

1       I    '      y  i                             1^          '              ■     ■'    ;       , 

!         1                                                  ■                         ' 

1    :  !    '  .                      '  i          1  1    1  i 

~r~^ — 1^      -                                  M  1  i~     ■ 

1   1       ■   !      1   1                                   .1                   1      1 

1825!                                 1826^                           ,1827 

,8?a   1                                 1829, 

_u^                                             f          1     i                                                  111 

itr      "■       XSl               _L  1!    . 

U '             1       1     !  '           1  1     li 

±                '^"-J^  \    -. .  _^i   LL  - 

i^^^-isi23^i±i,^ 

,  —-- ^  "\  -TN /^^  r\  -  - , ^      J- ' ^ 

^^ ,_._^li/L...  .>'^_^.v'._j_'!sAi^Z^ 

1  ,  . 

1        1    i        '                                   i            [                           ' 

'  [ 

-  -i-  4--L  -A  .    '  --4  '  -Li  -!_          -1- 

11      \  ■•      ill      1 

1830                                   1831                                   1832 

1833           j                1        1    IH..?*  ,        j 

i                  1                 i   ' 

i  ]     '  '                       !     hi     '  1 

,   A                 /V      M 

/  \        '  /^                          /  \ 

y     v^S.  A     1      :  .  /  N  ■  ..J      '  . 

rjT^s  z""^-   ;/:/  v^f/C/:^/^^'^" 

-t^  v   \/  iS-,  K  y\'   \  y^^t- - ■^. 

SjT"      '^'^ .-^      ^.        r      v*s/V^                     

i  ^^-^  ,  1  ><   .\-/^ 

— ' 

1          ' ' 

j 

1        !                       '1 

1838  t-     -    ■     -■    --j  1839                                I 

1 

:.    ....:!...  ■....             1     j  I"  1^  "^  ;  ., 

1                                     I 

0      1                                         1 

i                       '1                       11                            '         i 

1  i          1                    >                                     ^  <^                                  ' 

■    ■                              /  \                           ' 

^.j/\  .  .  /K        ,^^,  ■ ;  i/v  ■■ 

vjA/^'^N.V   Vr    A.  vA    i_ /\/> 

^"^^HW^S:t?^^Sfc 

„V^/^         V/        M       .     >/v/\     — /     s/ 

-6-.-^                  \t-              ^^                     -\^ 

1       f 

itlt                                         i 

1    f                                                                          < 

n^i^ 

irt40                            la^i                             ift4.? 

1 843 .                              ,1 844  ^ 

ill                             1 

1    1        I                             ' 

^        j                         1    '                                                        — ""^ 

J                                                                     1 

/^        '       '    '                  '    '                    J             / 

/\                       y''^                    y    ^^     'J—              ) 

/'       'S!^-^  xk    .  — ,           -.         !•  ""^ \      /       "^  V 

■."^-^"S         f    \                h"             -     /^  -"""'''''^          V    ^~"  \          ^ 

/                        \/      \V          \     /        1     \j                       ^ 

\  /      \       i\     f                  "^            ,                1             ^  —  /^ 

1  1    ^"v'^             ' 

1        \'/\/'        ''        li        1            ' 

1          v      V  ;                       1 

'  L             -u 

1        t    >                                   1        1 

1845^                              1846                                   1847 ,_ 

it  1848           It:     it        1849, 

[ 

1 

V                             .y . 

' 

i-^^^i^i'xz  .^±,zi  .  ^\.j\^^^L:lt. 

Ji  \>-'-^:^-\;y  /-''■'■s--''^/^--^   ^^ 

'     -T,.' -      -».^    ■•■>/>/ y/      Lr/>V  '•"    *■ 

i  1    '              >/  \/      ^.'                NF- 

1  I 

1                                                  ■ 

1 

I  1                                            1 

T    . 

I850+-                             1851a_                              1852 

1853                                         1854' 

"T 

1 

1 

I                                 1                              A 

iH  /-s  y    ^-N>v.y-sr  i--L 

^    .\/^A,r-A,    iA   i>-^\    ;>     —' ^'^     V 

iv^/-s>.'    -     +---v^s^      ^    /  aTS 

j    i           1 

1       1 

1 

1       1 

'SoSjI                             1856                                   1857' 

1858                                  1859  |_ 

- '  3^  =E      "J        i 

i                    \                                    'A 

^l                    - 

\        A             -             \ 

J      i                   V                       ^»       2l 

V   7\  .--/  ^1          V/   '^    .  -^     /V 

.         £          i         -V2\         ^\rJ--^'^^-~               -i^- 

Vy      H"       ^              I          ..  y\/..  \^..-S^..    1.1- 

-,,,(-  4^  ^^  ^-■'^c       ^:=-'^A^ 

T^              1    /               w   ^A 

,J-'                \' 

111 

i  1 

1 1 

VOLUME  2,    PLATE  VI. 


1819 
Oct.    Jam    Apr.    Juiv    Oct     .lAr^j 


DEPARTURES     OF    MEAN     MONTHLY     TEMPERATURE    FROM    NORMAL 

FOR    87    YEARS. 


MARYLAND  WEATHER   SERVICE.  VOLUME  2,   PLATE  VI. 

I860  1861  1862  1863  1864 

JAN.    Apb.  jul^    Oct.   jam.    Apr    July  Oct.    j^n    Apb    July   Oct.   Jan.    Apr.  July  Oct.    Jan.    Apr.  July    Oct.  ja 


1865 


88 


THE    CLIMATE    OF    BALTIMORE 


true  frequency  of  the  larger  changes  of  10°  and  above,  while  the  smaller 
changes  are  below  the  true  frequency;  hence  changes  in  the  daily  maxi- 
mum temperature  are  not  a  safe  guide  to  the  diurnal  variability  in  the 
geographical  horizon  of  Baltimore.  In  all  cases  the  changes  based  upon 
observation  of  the  maximum  temjjerature  from  day  to  day  differ  most 
widely  from  those  based  on  changes  of  the  average  daily  temperature.  In 
Fig.  21  and  in  the  following  table  these  results  are  shown  graphically  for 
changes  under  6°,  for  6°+,  8°+,  10°+  and  20°+,  when  the  diurnal 


\ 

\ 

\ 

/ 

\ 

\ 

\ 

\ 

\ 

\ 

/ 

\ 

^ 

\ 

/ 

I 

\ 

\ 

\ 

\ 

> 

/ 

20°+  10^  bV  eV         -0 

Fig.  21.— Diurnal  Changes  of  Temperature  of  —6°,  6°  +  ,  8°  + ,  10°  + ,  20°  +  . 

Fig.  21  shows  the  frequency  of  stated  changes  in  the  temperature  from  daj-  to  day  when 
based  on  the  minimum  temperature  of  two  successive  days,  on  the  mean  temperature,  on 
the  8  a.  m.,  on  the  8  p.  m.,  and  on  the  maximum  temperatui-e.  respectively.  See  also  Fig.  20, 
and  Table  XVII.  The  frequency  is  indicated  bj'  the  line  of  figures  above  the  diagram,  the 
degree  of  change,  Vjy  the  line  of  figures  below  the  diagram. 

variability  is  based  on  observations  of  the  maximum,  the  minimum,  the 
8  a.  m.  and  8  p.  m.  readings  and  on  the  true  daily  mean.  Diurnal  changes 
were  computed  for  a  period  of  30  years  from  the  daily  average  tempera- 
ture, and  for  a  period  of  10  years  from  the  maximum,  minimum,  the 
8  a.  m.,  and  the  8  p.  m.  observations. 

FREQCENCV  OF  DIURXAL  TEMPERATURE  CHAXCiES  OF  STATED  AMOUNTS. 
(Expressed  in  terms  of  departures  from  the  frequencj-  based  on  changes  in  the  daily 

mean  temperature.) 


Temperature  changes.  Minimum. 


Departure. 

Below  6° +11 

6°+ -    5 

8'+ i  -  10 

10'+ -    9 

20°+ '  -0.4 


Mean. 


241 

119 

71 

41 

1.7 


Departure. 


—  16 
+  19 
+  21 

+  18 

+2.7 


•  p.  m. 


Maximum. 


Departure. 


—  21 
+  21 
+  20 
+  18 
+2.3 


Departure. 


+  51 
+  48 
+  42 
+6.6 


MARYLAND    WEATHEE    .SERVICE 


89 


The  smaller  changes,  under  G°,  increase  in  frequency  very  rapidly  from 
February  to  July,  then  decrease  at  a  similar  rate  to  February.  Changes 
of  6°  and  over  occur  most  frequently  in  the  months  of  December,.  Jan- 
uary, February  and  March ;  the  decrease  is  then  uniform  until  a  minimum 
frequence  is  reached  in  August :  then  there  is  a  more  rapid  increase  to 

TABLE  XVIir.-FIVE-DAY  MEANS  OF  TEMPERATURE. 
(For  five-day  periods  ending  on  given  daj-s.) 


January. 

February. 

March. 

April. 

May. 

June. 

5th      33.3 

4th       33.5 

1st      .37.0 

5th      48.4 

5th      59.9 

4th       70.4 

10       33.3 

9      as. 6 

6        38.1 

10       50.1 

10       63.9 

9       71.6 

15     as.s 

14       35.7 

11       40.9 

15       52.5 

15       6:^.7 

14        72.6 

20       33.9 

19       36.6 

16       40.8 

30       53.9 

20       64.7 

19        73.8 

25       34.3 

24       36.9 

21        41.0 

25       56.5 

25        66.1 

24        75.5 

30       33.5 

26       41.8 
31        44.6 

30       58.1 

30       67.9 

29        76.5 

Jul}-. 

August. 

September. 

October. 

November. 

December. 

4th       76.7 

3rd      76.8 

2nd     72.7 

2nd      63.0 

1st      .51.8 

1st     3S.7 

9       77.7 

8       76.8 

7       72.5 

7       61.3 

6       49.5 

6       38.4 

14       78.0 

13       76.9 

12       70.1 

12        58.7 

11       49.1 

11       3S.9 

19       78.8 

18       75.4 

17       68.4 

17       .57.6 

16       46.0 

16       38.3 

24        77.2 

23       75.2 

23       66.6 

22       55.4 

21        43.9 

21       35.6 

29        77.7 

28       73.6 

27       65.0 

27       53.9 

26        42.3 

26       38.3 
31       33.7 

TEN-DAY  MEANS  OF  TEMPERATURE. 

(For  ten-da  J-  periods  ending  on  given  days.    Derived  from  above  table  of  five-day  means.) 


January. 

February. 

March. 

April. 

May. 

June. 

10th     33.3 
30        33.6 
30        33.9 

9th      33.5 
19       36.2 

1st      37.0 
11       39.5 
21        40.9 
31        43.2 

10th     49.3 
20       53.2 
30        57.3 

10th     61.4 
20        64.3 
30        67.0 

9th      71.0 
19       73.2 
29        76.0 

July. 

August, 

September. 

October. 

November. 

December. 

9th       77.2 

19       78.4 
29        77.5 

I 

8th       76.8 
18        76.2 
28       74.4 

7th      72.6 
17        69.3 
27        65.8 

7th      62.1 
17        58.2 
37        54.6 

6th      50.7 
16       47.5 
26       43.1 

6th    33. 5 

16    38.5 

26    36.0 

Jan.  5  33.5 

Table  XVIII  shows  the  mean  temperature  for  each  successive  period  of 
five  (lays  beginning  with  January  1st,  and  also  for  each  successive  period 
of  ten  days.  The  5-day  and  10-day  means  were  computed  from  the  normal 
daily  temperatures  for  the  30-year  period  1871-1900,  after  reducing  the  latter 
to  the  true  daily  temperature  based  on  hourly  observations. 


December.  A  change  of  20°  in  the  average  temperature  of  two  successive 
days  has  occurred  at  Baltimore  about  50  times  in  the  30  years  from  1871 
to  1900.  Of  these  occurrences  15  were  recorded  in  February,  10  in 
January,  8  in  December,  5  in  ^larch,  5  in  November,  2  in  each  of  the 

7 


90  THE    CLIMATE    OF    BALTIMORE 

months  of  Aprils,  May,  and  October,  none  in  the  months  of  June,  July, 
August  and  September.  The  most  frequent  change  and  hence  the  most 
probable,  is  a  change  of  2°  in  the  spring  and  summer  and  3°  in  the 
autumn  and  winter  months. 

The  Probable  Error  of  the  Meax  Daily  Temperatures. 

No  law  has  yet  been  discovered  governing  the  departures  from  the 
normal  temperature  of  a  year,  month,  or  day.  Departures  above  and  below 
the  normal  for  a  long  series  of  observations  agree  very  closely  in  their 
distribution  with  chance  occurrences.  Hence  the  formula  applicable  to  the 
latter  has  been  employed  in  the  determination  of  the  probable  error  of 
average  temperatures  for  a  given  period. 

The  equation  used  for  finding  the  probable  error  of  the  daily,  monthly, 
and  annual  means  of  temperature  for  Baltimore  is  a  form  suggested  by 
Fechner  *  and  is  as  follows : 

J.  ^  J.     1.1955 


^271  —  1 

in  which  E  is  the  probable 

error,  v  the  average  departure  from  the  normal  temperature  (in  degrees 

Centigrade)  not  regarding  the  sign  of  the  departure,  and  n  the  number 

of  occurrences,  in  this  case  the  number  of  years  of  observation.     The  value 

1.1955 
of  the  factor     .'.         ^   is  as  follows  for  the  stated  periods  of  observation : 
\/27l  —  1  ^ 

20  30  40  50  60  70  80  90      100  yrs. 

0.191  0.156  0.134  0.120  0.110  0.102  0.095  0.089  0.085 
In  order  to  determine  the  probable  error  of  the  daily  mean  temperature 
at  Baltimore  for  the  30-year  period,  from  1871-1900,  the  above  formula 
was  applied  to  a  representative  day  in  each  season,  namely,  for  the  15th 
day  of  January,  April,  July  and  October.  In  winter  (represented  by 
January  15),  the  average  departure  v  of  the  mean  daily  temperature  from 
the  normal  is  7°,  in  spring  (April  15),  5°,  in  summer  (July  15),  4°,  in 
the  autumn  (October  15),  6°.  In  individual  cases  these  departures  vary 
greatly.     On  January  15,  1871,  the  mean  daily  temperature  was  62°, 

*See:  Hann's  Lehrbuch  der  Meteorologie.     Leipzig,  1901,  p.  107. 


MARYLAND   WEATHER    SERVICE 


91 


or  28°  above  the  normal  value;  on  January  15,  1893,  the  mean  tempera- 
ture of  the  day  was  22°  below  the  normal.  Thus  the  loth  day  of  Jan- 
uary shows  a  range  in  the  average  temperature  of  the  day  of  50°.  The 
extremes  on  April  15th  were  17°  above  and  11°  below,  a  range  of  28°  ;  on 
July  15th  6°  above  and  12°  below  the  average,  a  range  of  18°  ;  on  October 
15th  the  extreme  departures  were  plus  15°  and  —  19°,  a  range  of  34". 
These  figures  strikingly  illustrate  the  variability  of  temperature  conditions 
within  short  periods  at  Baltimore. 

THE    FREQUENCY   AND    AVERAGE    VALUE    OF    DEPARTURES    FROM    THE 
NORMAL  DAILY  TEMPERATURE. 


January  15th. 

April  15th. 

July  15th. 

October  15th. 

Departures. 

Departures. 

Departures. 

Departures. 

Fre- 
quency. 

+18 
-15 

Sums. 

+  116.6° 
-116.5° 

Aver- 
age. 

+6.5° 

-7.8° 

Fre- 
quency. 

+13 
-20 

Sums. 

+84.9° 
—84.0° 

Aver- 
age. 

+6.5° 
-4.2° 

Fre- 
quency. 

+20 
-13 

Sums. 

+61.0° 
—60.2° 

Aver- 
age. 

+3.0° 
-4.6° 

Fre- 
quency. 

+17 
—16 

Sums. 

+106.6° 
—105.2° 

Aver- 
age. 

+6.3° 
-6.6° 

33 

233.1° 

7.1° 

33 

168.9° 

5.1° 

33 

121.2° 

3.7° 

33 

211.8° 

6.4° 

Entering  the  values  of  the  average  departure  v  in  the  formula  we 
obtain  as  the  probable  error  of  the  mean  temperature  of  a  typical  winter, 
spring,  summer,  and  autumn  day  the  following  values: 

January  April  July  October  Average  Seasonal 

1.1°  0.8°  0.6°  0.9°  0.8° 

These  figures  represent  the  probable  error  of  a  daily  mean  temperature 
in  the  respective  seasons  for  a  series  of  observations  at  or  near  Baltimore 
covering  a  period  of  30  years.  The  daily  mean  temperature  will  not  be 
increased  or  decreased  by  an  amount  greater  than  these  values  by  extend- 
ing the  series  of  observations. 


Mean  Monthly,  Seasonal,  and  Annual  Temperatures. 
There  is  an  excellent  series  of  local  temperature  observations  extending,, 
with  very  few  interruptions,  from  1817  to  date.     For  the  series  from  1817 
to  1824  we  are  indebted  to  Captain  Lewis  Brantz,  who  kept  a  careful 


93  THE    CLIMATE    OF    BALTIMORE 

record  of  the  weather,  in  what  was  in  his  time  West  Baltimore,  and  pre- 
sented his  published  results  to  the  Mar5dand  Academy  of  Sciences,  of 
which  he  was  a  member.  His  observations  were  made  at  five  stated 
hours  of  the  day,  at  sunrise,  8  a.  m.,  2  p.  m.,  sunset,  and  10  p.  m.,  and  com- 
prised the  elements  of  pressure,  temperature,  wind-direction  and  force, 
clouds,  and  rainfall.  In  1831  systematic  observations  of  the  principal 
climatic  elements  were  made  at  7  a.  m.,  3  p.  m.,  and  9  p.  m.  at  Fort 
McHenry,  under  the  auspices  of  the  U.  S.  Army.  This  series  was  main- 
tained to  the  year  1892  with  the  exception  of  two  or  three  years  just  pre- 
ceding and  during  the  Civil  War.  From  1871  to  the  present  time  a  first 
order  station  of  the  U.  S.  Weather  Bureau  has  been  maintained  at 
Baltimore. 

To  complete  the  record  since  1817  it  has  been  necessary  to  interpolate 
observations  made  at  neighboring  localities,  applying,  however,  the  proper 
corrections.  This  could  readily  be  done  as  the  different  records  over- 
lapped. The  break  in  the  record  from  1825  to  1830  was  filled  in  by  re- 
ducing Washington,  D.  C,  observations  to  the  Fort  McHenry  series;  for 
the  years  1859  to  1863  the  excellent  record  maintained  for  20  years  at 
Shellman's  Hills,  about  20  miles  due  west  from  Baltimore,  was  utilized. 
A  year's  record  by  Captain  Brantz  in  1836  afforded  a  means  of  reducing 
his  earlier  observations  to  the  Fort  McHenry  series.  From  1871  to  1892 
the  Fort  McHenry  and  the  U.  S.  Weather  Bureau  records  overlapped. 
All  of  these  observations  were  ultimately  reduced  to  the  U.  S.  Weather 
Bureau  series  by  applying  the  necessary  corrections  and  were  thus  con- 
verted into  a  comparable  and  continuous  record  of  great  interest  and  value 
in  the  discussion  of  the  climatic  conditions  of  Baltimore  City. 

The  monthly,  seasonal,  and  annual  means  are  presented  in  Table 
XIX.  The  departures  from  the  normal  values  are  shown  in  graphic 
form  in  Plates  VI  and  VII.  The  table  and  diagrams  afford  excellent 
material  for  the  study  of  the  changes  in  temperature  conditions  exper- 
ienced by  Baltimoreans  during  the  preceding  century  and  incidentally 
the  results  throw  light  upon  the  assertion  of  the  "  oldest  inhabitant "  that 
our  winters  are  growing  milder,  an  assertion  which  has  been  persistently 
repeated  since  the  earliest  settlers  arrived  on  our  shores. 


MARYLAND  WEATHER   SERVICE 


VOLUME  2,  PLATE  VIL 
880  1885  leSO  .'8?? 1900 <aa= 


1820         182B  1830         1B35  1840  1845  18&0  185&  1860 

DEPARTURES    OF    MEAN    MONTHLY,    SEASONAL    AND    ANN 


865  1870  1875  1880  1886  1890  1895         1900  1905 

UAL    TEMPERATLIRE     EROM    NORMAL    FOR    87    YEARS. 


:marylaxd  weather  service 


93 


TABLE  XIX.— MEAN  TEMPERATURES  AT  BALTIMORE  FOR  88  YEARS,  18I7-190i. 


Years. 


1817. 

1818. 
1819. 
1830. 

1821. 
1822. 
1823. 
1824. 

1825. 

1826. 
1827. 
1828. 
1829. 
1830. 

1831. 
1832. 
1833. 
1834. 
1835. 

1836. 
18:^7. 
18;}8. 

18:». 

1840. 

1841. 
1842. 
1843. 
1844. 
1845. 

1846. 
1847. 
1848. 
1849. 
18.50. 

1851. 
18.T.2. 
1853. 
1854. 
1855. 

18.56. 
1857. 
1858. 
1S59. 
I8f». 

1W!1. 
lsfi2. 
18(^!. 
1864. 
1865. 

18(i6. 
1867. 
iHtW. 
1869. 
1870. 


33.230.143.461.962.072.2 
,  .35. 6:«. 942. 9 50. 861. 7 74. 5 
,40.739.0.39.754.5,63.475.9 

:«). 4  42.944.656.41.59.172.4 


,540, 
,6.36. 
,731. 
437. 
6,39. 

941. 
4  42. 
H4."). 
729. 
,033. 


249.062.877.0 
259.570.775.6 
,4.59.166.4:2.(1 
9.55.56:^.772.7 
a.56. 763. 876.1 


4.53.772. 
.■<(;0.3f!5, 

t;50.3i;2. 

1.56.763. 
8i56.664. 


.31.232.648.4  57.765.075.9 
,  34 . 3 .39 .345. 6 .53 . 9  63 . 5  72 . 9 
.  39.639.241.957.87n.sV3.s 
.  32.246.3  4H.3.56.5H1.,M73.1 
.34.330.942.1.50.264.772.3 

.36.327.933.9.52.764.167.9 
.  31. 335. 941. 950. 363. n;i. (I 
.  39.8  2S.6  43.7  49.660.n:.">.t; 
.34.936.544.41.57.666.971.1 
.26. 740.546. 4L55.4I62.2'72. 3 

..32.9-33.6  11.5  48.6.56.4  70.7 
.38.939.94'.t.].".5.4r,(i.:i:o.l 
.  40. 929. 931. 251. .-.M. 77:!.: 
.  31. 7;i3. 943. 057. 167. 270. 4 
.39.335.945.2.55.861.372.9 

!     '     i 

..34.831.4  43.1.54.2  65.569.2 
..3:5.234.339.1.56.961.971.] 
.  40. 0:!S. 041., s5-<.l(l,s. 1176.0 
.  .34 . 4  :J2 . 7  45 . 5  53 . 2  61 . 9  76 . 3 
.40. 741. 6143. 9.51. 861. 975. 8 

.  .39.841.348.1  55.965.572.5 
30.537.744.149.263.971.2 
,  34.8 :)8. 5 43. 9. 54. 4 65.0  75.1! 
,  36 . 1 :58 .345. 5 .50 . 2  65 . 0  7:5 . 1 
.  39.1,;50.740.6.58.965.572.3 


26 . 3  28 . 4  35 . 3  56 . 2  63 . 3  76 . 6 
25.94:i.l  tl.5  17.7<!:i.(i:2.5 
43. 9:!:!. 54:!. 455. 461. ;i77. 7 
37 . 3 :58 . 7  49 . 5 .54 . 2  64 . 9  70 . 7 
33. 7  ;K. 7  46. 2.52. 164. 9  70. 7 
(        I        I        , 

32.7:59.144.4.54.6.58.573.5 
:54.2:U.4:f9. 951. 561. 969.1 
:!6.2:i2.4:i.S.(l49.964.670.9 
:i7 . 9  40 . 4  4:; .  4  52 . 5  68 .  S  74 . 7 
;5:,' .  6  -m .  1  50 . 3 .59 . 1 68 . 2  77 . 3 

:S:i.4:!S.O  11.9.57.161.1  7:!. 3 
2«.5:i0.6:!9.,^.5,'^.:i(il.il7:!.7 
:52.9:iO.()4:i.l51.262.'<71.1 
,  40. M41. 0,42. 5.55. 962. 3  74. 5 
42.937. 741. 156.365.978.8 
I        I        I        I        I 


^     P 


77.9  74.8  67.353.8.50.2,38.2 
79..- 76. 865.6,53. 3 50. 6:33. 8 
77.779.8  71.0.51.550.2:^7.9 
77.7177.269.0.51.642.5:36.7 


75.6 
,^1.0 
7S.0 
79.1 


9' 

SO.;!' 

77.9 
74.9' 

81. of 


071 


.067. 
.8j68. 

.273. 

.471. 
.169. 
.066. 
.0170. 


3  55, 
7. "9, 
s.M, 
1.58. 
4|60. 

2.58, 

1  .-.s. 
2.54. 
1.54. 

2|.58. 


6  16. 

244! 
148 
3,46 

8  47 
5  46 

7  .50 
145 
8.54 


38. 

m. 
2  41. 
044. 

537. 

7:57. 
S43. 
441. 
,545. 
,4;38. 


76.676.967.4.59.414.727.0 
67(1.771.959.4  17.S40.6 
275.1  69. 6.-,4.:{45.:540.1 
8l.578.;5!67.]:.52.6|45.6  3-<.l 
i6.6j73.5l62.1|,58.0|49.434.9 

r5.9'71.06S.S47.S42.ri34.2 
:5.,s;i.,^<14.S.V).947.737.9 
~1. 7  :,-^. 467. 9.50. 941. 7:52. ■; 
i8.273.967.4|.59.7i41.2,;i5.9 
r4. 9175.5163.8155. 3144. 7i31. 5 

r7. 575. l.0. 948. 943.1:56. 7 
:ii.574.46,s.4.-i4.o:!;i.<t:i4.:> 
,'6.,s77.471.5.'i4.04:i.o:i6.,s 
8.5  75.1  66.9.52.542.1:54.8 
'7. 2!76.4i67. 1.55. 9)46. 430.1 


5.4 


,270. :!.-,:;. n  1^ 


:!r.i 


;'6.477.2ii.-,.,-,,V.  .2i:;.2  15..-, 
1'7 . 3  76 . 5  6,s. :.'  :Vi .  7  54 . 6 :5,s . ', 
1-9.7175.0168.21.57.81.52.4142.0 

r9 . 4  74 . 6 119 . 7  58 .517.6  :S3 . 9 
r5. 9:7. 066. 2.57. 944. 042.1 
1 7. 176. 2  70.1. 5:!. 7  49.7:58.2 
r9. 4  76. 4  71. 6.57. 951. 6:55. 4 
r9. 176. 9170.2)63. 9149. 6'39. 4 


sn. 


').0(1,> 

■,.411,- 


5  17.2:55.9 
916.  t  !:;.-< 
;;i.l  ■.:,.si\:.s:,s.',  c;.'.)  11. ii 
76.2  75.168.0  52.4  48.4:52.8 
76. 7;75.8|65. 11.56.2145.6,33. 8 

74.872.769.1.59.144.737.8 
74.675.96'.l.:!.",s.l  12.,'<:!6.7 
77.1  77. H64.:!.".2. 7  46.2:51.:! 
78. ;!i79. 569. 557. 248. 5:57. 7 
79.476.174.1.53.147.8:59.8 

79.1  71. 670. .K.5S. 151.5:55.6 
77.9",6.:iii!t.6.5-<.(i49.'.  :!4.4 
,s:  i .  2  ;  9 .  (1  71 1 . 0  55 .  ,><  4.>< .  4  34 . 5 
78.176.6,70.15:3.143.240.5 
83. .5j80. 671. 1159. 4148.4137. 5 


<^ 


55. 4| .55.8 

.54.6.34.951.8 

56. 7,-37. 81.52. 5 
.55.037.153.4 


g.0|-57. 11817 
6.91.56. 511817-8 
77. 8i57. 611818-9 
5. 8i54. 411819-20 


.55.. 1.35. 


437, 
.340. 
.6:40, 

I 

.538, 
.4:57, 
.24:!, 
.2:^5. 
.6|.38, 


].51.077.9.57.91.<«20-1 
4."9.]78.7  61.61S21-3  ' 

156.6  76.0  55.71822-3 
7. 54. 4i75. 6.57.71823-4 

4.56.6[77..5[58.4jl824-5_ 

9.58.4  77.1.59.9182.5-6' 
6 .■.7.7  77. 9. 5,-^.^1826-7 
:>.":).,^7.s.4.5.s.Jl,S27-8 
1 . 54. 2|75. 1.55. 21828-9 
41.55. 8177. 9161. 11829-30 


.55.1:54.2.57.0176.5.57.21830-1 
56 . 9 :5:! .  5 .54 .  ;5  75.7  59 . 7  1831-2 

57 . 1  39 . 8  56 .  S  75 . 7  .56 . 4  1832-3 
.56.7:59.5.55.577.6.55.118.3.3-4 
.54. 0,34. 4|.52. 3174. Ij56., 511834-5 

.51. 8:53.0  50.2  71. 6. 5:5.1  183.5-6 
54.l:!:i.s5].77:i.!i5(;.l  ls:56-7 
.^4.i:>5.t5].i;s.6."M.5i,s:s7-8 

.55.6:34.7.56.;5i74.4  56.]il8;-58-9 
53 .9  34 . 4  54 .  7j74 .  21.54 .  61839-40 

.52.9.32.748.8  74.4  54.31840-1 
55.l):is..-,,54.9^:!.';  .54. 11841-2 
.'3. 9 :!5. (I  44.,-.!  76. 0.56. 2  1842-3 
54 . 4  ::54 . 1 1 .55 . 8: 74 . 7 .53 .  81843-4 
55. 2i.36 .  7154 . 1 175 .  ,5.56 .  511844-5 

i       j       !       i       i 

.■4.7:52.1.54.373.357.5184.5-6 

55. 6 :i4 . 6 .53. 6  75. 4  -''S- 1 1846-7 

57 . 2  :!9 . 2  .56 . 2  ■;  6 . 5 .55 . :!  1847-8 
.56. 2::57. 5.53. 5  76. 7.59. 8 1848-9 
.57. ,5[40. 3152. 5i76. 81.59. 5 1849-50 

.57.2  41.0.56.575.5.58.618.50-1 
,'4 . 9  :!1 .  ( 1 .52 . 4  74 . 7  56. (1 18.51-3 
.51  i .  4 :  ;,-< .  5  ,",4 . 4  ■;  6 . :!  .57 .  M852-3 
.56. 6:57. 5. 5:5. 676. ;i60. 4  18.53-4 
.57.ip5.1|55.0j76. 1161.21854-5 

,'4.131.4.51.6  77.5.57.5185.5-6 
,55.ii:i5.ll.50.7';i.,'^,57. 1  18.56-7 
.56.,s4(l.4  .5:>.4  77.5.56..S  ly,-)7-8 
.55.6:59.3 ,56.2  74.U.56.0 18.58-9 
54 .  41.33 .  11.54 .  4|74 .  41.55 . 6 1859-60 

i      I      !      I      i 

55 . 0  35. 2 .52 . 5  73 . 7  57 . 6 1,860-1 
.•3.';  :i4.."51.1  7:5.256.71861-3 
.':!.6:!5.1  .■rt)..s75.:).54.4  1862-3 
.57.3:57.554.9  77.5.58.4186.3-4 
.58.235-5159.277.6160.01864-5 
i        i        ■        I 

56 . 4 .37 . 1  55 . 4  74 . 7  60 . 1 1.86,V6 
.55. 7:51. 6.5:!. 2711. 059.:!  186(3-7 
.55.3:12. 1 .52.4  7,'<..s.5.s.]  1S67-8 
.56. fi:5,S.,s.>5. 676. 4.55. .51868-9 
.58.  £  40 . 4  54 . 4  81 . 0 .59 .611869-70 


94 


THE    CLIMATE   OF   BALTIMORE 


Table  xix  Con't.— MEAN  TEMPERATURES  AT  BALTIMORE  FOR  8S  YEARS,  1817-1904. 


Tears. 


1873 

1873 

1874 

1875 

1876 

1877 

1878 

1879 

1880 

1881 

1882 

1883 

1884 

1885 

1886 

1887 

1888 

1889 

1890 

1891 

1892 

1893 

1894 

1895 

1896 

1897 

1898 

1899 

1900 

1901 

1902 

1903 

1904 

•O  '  _. 


fe    S  h^  f  s 


. .  .35.■3'.39.847.758.96.^.474.0 
..!34. 436. 0.37. 055. 367. 275. 3 
..  34. 2!.35. 640. 4 52.062.3 73.9 
..39.3,37.443.,S47.063.175.9 
.  .130.329. 439. 649. 564. 173. 7 

..i41.7i37.9j39.9,.52.264.2i75.9 
..132. 340.614]. 553. 762. 773. 
.'34.9  40.4  49.2  58.6^3.570.2 
. .  .31.5.'J2.243.S52.S65.7'73.3 
.42.9  41.2  42.855.170.875.3 

..130.334.641.8.51.767.870.8 
..,34.741.345.052.1.59.674.0 
..|32. 339. 1.39. 552. 264. 274. 6 
.  132.042.244.1.52.465.073.2 
..|34. 0:28. 535. 4. 54. 363. 272. 6 

..;29. 131. 341. 7.54. 7162. 169. 9 
..132.4.38.237.9.51.367.672.3 
..  129.1.35.. 537. 4.52. 7  62. 8  73. 2 
..138.730. 644. 1.54. 8165. 771. 2 
..43. 7:43. 040. 9.53. 963. 775.1 
I        , 

..37.341.639.0.56.0162.1171.7 
..31.936.237.3.51.963.175.5 
..24.334.040.4.52.561.672.5 

. .  37.434.447.S.''i2.364.W73.1 
..31.4  26 . 2  40 . 2  52 . 8  62 . 5  74 . 3 

..133.335.237. 8.56., 568. 7  71. 3 

..130.8.36. 6  45. 2. 52. 66;^. 4  69. 9 

}6.7;34.7;48.6.51.46:3.573.7 

!2.7as.041.6r)3.664.875.3 

35.333.138.6.54.864.9  72.9 

34. 428. 742. 951. 062. 0'73. 5 
31 .  .529 . 8  46 . 2:.53 . 2,63. 9!71 . 9 
.33. 536. 9149.7  54. 8165. 5167.5 
27.538.641.350.5165.871.6 


Ten-Tear  Mean. 

1821-1830 

1831-1840 

1841-18,50 

18.51-1860 

1861-1870 

1871-1880 

1881-1890 

1891-1900 

Gen'l  Averages. 

1817-1870 

1871-1903 

1817-1903 


35.737. 043. 653.5j64. 9 
33.6136.440.8.53.064.2 
33.134.041.653.463.9 


.36.337.746.155.765.575.0 
34.135.843.7.54.264.272.6 
36 . 7  35 . 1  42 . 3  .'i  4 . 3  62 . 0  72 .  ( i 
34. 7  36. 3  43.  S  53. 4  64. 2  73. 3 
35.2|35.742. 754.663.9  74.0 


35.4 


34.9 


36.1 


34.035.5 


35.8 


43.6 
42.1 
43.1 


54.6 
53.3 
54.1 


63.8 
64.3 
64.0 


74.1 

73.7 
73.0 


73.5 
73.1 
73.3 


76.0177.563.758.745.233.8 
81.7|79.869.5.56.943.7|32.4 
■9. 676.668. 3.55. 341. ,5;40 
■7.573.1  70. 21.57. 145. 8j39. 3 

■8.273. 766. 2:,56. 043. 438. 6 

80 . 6  76 . 2  65 . 9'52 . 9  47 . 6  29 . 0 
8.977.968.260.24S.443.,s 

80.876.069.71.58.847.4  35.6 
9.075.365.263.046.741.9 
■7.7  74.7|68.3|,56.O42.031.3 

■8.977.2'77.2:a3.,549.3;43.8 
6.9  74.269.361.944.5:^.7 
'7. 173. 265. 41.57. 94S. 430.0 
5. 475. 572. 4,60. ti  Hi. r,;;;.  t 

■9.974.9,67.2:55.8  t:,.,s;i;.ti 

4.773.869.91.59.046.6131.3 
0.673.665.01,56.345.3137.2 
4.676.565.11.53.048.337.2 
6.573.766.3.54.147.945.1 
6.374.167.8.57.047.534.9 

71. 6(74. .5'70.5|54. 4143. 9143.0 
■6. 8j76. 4:66. 7, 56. 044. 0133. 7 
■6.875.665.9157.1  43. 9'38.] 
■7.473.769.9.57.443.337.3 
3.1  77.271.71.53.4  47. 03S 

7.7  77.068.0,54.3.50.936.2 
7.074.968.9.58..5'46.238. 
8.677.771.2.59.044.6.36. 
77.1  76.066. 5,59. 046. 736. 8 
9. 680. 373. 562. 0,49. ,5,36. 6 


9.876.9168 
77. 274.1167 
77.373.767.7 
75.7. 


.56.1141.8134.7 


58.6 
58.7 


51.735.3 
43.333.9 


78.678.069.8.57.348.3140.8 
7.775.467.1,55.345.135.3 

7.475.96,s.(i.54.ii4(i.;):!7.5 
,s.0T;-).>fi,s.(l.-,7.14;.437.7 
8. 676. 669. 8157. Oi47. 136. 9 


79.0 
77.1 
76.6 


78.1 
77.6 
77.9 


76.1 
74.7 
76 


369 


76.4 
75.6 
76.1 


67.5 
68.6 
3 


57.5145.236.6 
.57.947.038.0 


57.146.0 


56.0 

57.5 


.37.5 


37.7 

37.1 

56.6146.637.4 


47.0 
46.0 


,56.237.5.57.375.8.55.91870-1 


34.7,53.3 
•55.034.1,51.6 
■55.8,39.1.51.3 
•53.533.0,51.1 

■55.3.39.4,52.1 
.56.8.34.0-52.6 
•57.]  39.7.57.1 
•55. 8a3. 1.-4.1 
•56.442.0.56.2 

•57.232.1  .'S. 8 
•55.8.39.9.52.2 
.55.236.0.52.0 
56.337.7  r3.« 
.54.0:33.3.51.0 

.^^3.6132. 7!  .52. 8' 
,':4.7,34.0,52.3 
,'■4.133.951.0 
55.635.5.-4.9 
.56.4  43.9,52.8 

•55.437.9.52.4 
."4.037.0.50.8 
.'3.4,30.7.51.5 
.55.6.36.6.55.0 
•54.031.6.51.8 

•55.. 5  35. 7  •■4. 3 
.55.234.5.53.7 
.56.2.36.6.-4.5 
•■4.832.3.-3.3 
.56.635.1.52.8, 

.54.1*3.2.53.0 
■55. 033. 0.54. 4' 
.54.935.2)56. 


■8.9.56.7:1871- 
6.7 .55^5 1872-3 
■5.5.57.71873-4 
■5.2.55.21874-5 

'7.6.55.5187.5-6 
■6.8.58.91876-7 
■5.7.58.61877-8 
5.9.58.31878-9 
■5.9.55.41879-80 

■5.863.31,880-1 
■5.0,58.61881-3 
5.0,57.31883-3 
4.7-59.91883-4 
5.8.56.31884-5 

73.8|.58.5il88.5-6 
■5. 5155. 51886-7 
■4.8.55.51887-8 
3.8.56.11888-9 
5.3  57.41889-90 

■2.61.56.31890-1 
•6.2.55.61891-2 
'5.0-55.6  l,s92-3 
■4.7.56.91h9,3-4 
■4.9.57.41894-5 

■5.3.57.7.1895-6 
3. 9 .57. 9: 1896-7 
■6.71.58.31897-8 
■6.1.57.41f*9,S-9 
■7.6:61.71899-00 

■6.7  55.51900-1 
4.4.59.31901-2 
2.5(56.61902-3 


29.7i52.5l....l 1903-4 

Ten-  rear  Means. 

57. 4|38. 3.55. 8177. 2'58. 5 1821-1830 
.55.0!35.]  .^4.0|75.2.55.8 1S31-1840 
.55.;!:i6.4.52.9  75.3  56.51,«41-18.50 
.55.,s;iti.:.'.-3.,S:75.7.57.7).S,5l-1860 
56.0,35.9,53.7  76.4.58.01861-1870 


.55.836.4,53.7 
55.336.0.53.7 
.55. 134.9  i3.0 


55.936.4 
.55.335.5 
55.636.0 


54.0 
.^3.2 
53.7 


76.456.7:1871-1880 

74.8157. 81881-1890 
75.3.57. 5J1891-1900 


76.057.2  1817-1870 

75.457.31871-1903 

5. 8  57.31817-1903 


Table  XIX  shows  the  mean  monthly,  seasonal  and  annual  temperature  for 
Baltimore  for  88  years  from  1817  to  1904.  The  table  contains  three  distinct 
records:  (a)  observations  from  1817  to  1824,  by  Capt.  Lewis  Brantz,  in  what 
•was  then  west  Baltimore;  (b)  observations  at  Fort  McHenry,  along  the 
Baltimore  harbor,  from  1831  to  1870;  (c)  observations  under  the  auspices  of 
the  United  States  Weather  Bureau  from  1871  to  1904. 


MARYLAXD    WEATHER    SERVICE  95 

The  Lewis  Brantz  observations  were  reduced  to  the  Fort  McHenry  series 
by  applying  corrections  derived  from  an  overlapping  period  in  1836  and 
1837.  The  monthly  means  for  the  period  from  1825  to  1830  were  derived  from 
Washington,  D.  C,  observations,  and  reduced  to  the  Fort  McHenry  series  by 
adding  the  departures  from  the  Washington,  D.  C,  normal  temperatures  to 
the  Fort  McHenry  normal.  The  means  for  the  years  1859  to  1863  were  re- 
duced to  the  Fort  McHenry  series  in  a  similar  manner  by  means  of  a  20-year 
record  of  overlapping  observations  made  at  Shellman's  Hills,  about  20  miles 
west  of  Baltimore.  The  record  from  1817  to  1870  was  then  made  conformable 
to  the  Weather  Bureau  record  from  1871-1903  by  means  of  departures  derived 
from  overlapping  records  covering  a  period  of  about  20  years.  Thus  the 
entire  record  from  1817  to  1^03  may  be  regarded  as  an  approximately  uni- 
form series  of  Baltimore  City  temperatures. 

THE   XORMAL   TEMPERATURE. 

The  variations  of  the  mean  monthly,  seasonal  and  annual  temperature 
during  the  greater  portion  of  the  preceding  century  are  discussed  in 
succeeding  pages,  while  the  mean  for  each  month,  season  and  year  since 
1817  is  published  in  Table  XIX,  together  with  the  monthly,  seasonal,  and 
annual  averages  for  each  ten-year  period,  and  for  the  entire  87  years. 
The  variations  in  value  of  the  ten-year  averages  are  observed  to  be  small, 
even  in  the  case  of  the  month  of  greatest  variability.  The  maximum 
variability  (5,3°),  occurs  in  the  month  of  March  with  a  mean  tempera- 
ture, for  a  ten-year  period,  as  low  as  40.8°  and  as  high  as  46.1°. 

The  annual  average  for  ten  years  has  varied  between  the  limits  55.0® 
and  o7.-4°  a  range  of  2.4°.  Xo  progressive  increase  or  decrease  is  indi- 
cated for  the  entire  period  eitlior  in  tlic  monthly,  seasonal,  or  the  annual 
means.  From  the  third  decade  (1831-1840),  there  was  a  steady  rise  in 
temperature  to  the  sixth  (1861-1870),  and  since  then  a  steady  fall  to 
the  present  time.  The  series  of  observations  is  not  sufficiently  long, 
however,  to  draw  the  conclusion  that  there  is  a  periodic  change  of  this 
length.  In  the  absence  of  changes  of  long  period  in  the  fluctuations  of 
tlie  annual  mean  temperature  the  normal  temperature  derived  from  the 
87  years  will  remain  fixed  at  55.0°  for  Baltimore.  The  probable  error 
of  this  value  is  not  greater  than  one-tenth  of  one  degree  Fahrenheit. 
Hence  a  longer  series  of  observation  will  not  cliange  the  result  by  an 
amount  greater  than  onc-tentli  of  one  degree. 


96  the  climate  of  baltimore 

The  Vakiability  of  the  Monthly  and  Annual  Mean. 

In  tabulating  the  following  lists  of  exceptional  months  and  seasons  the 
sole  basis  of  selection  has  been  an  average  monthly  temperature  decidedly 
above  or  below  the  normal  for  the  entire  period  of  87  years.  Such  lines 
of  division  must  necessarily  be  arbitrary  as  there  are  no  fixed  standards  of 
cold  and  warm.  The  degree  of  departure  from  the  normal  fixed  upon 
for  classification  as  cold  or  warm  varied  with  the  variability  of  the  month 
and  season.  In  the  comparatively  constant  summer  months  a  departure 
of  2°  may  be  regarded  as  exceptional.  In  the  variable  winter  months  a 
departure  of  6°  may  be  assumed  to  be  necessary  to  make  the  month  an 
exceptionally  cold  or  warm  one.  The  seasons  and  the  year  being  less 
fluctuating,  the  departures  selected  were  smaller,  varying  from  2°  for  the 
year  and  the  summer  to  4°  for  the  winter  season. 

A  close  examination  of  this  list  of  exceptional  departures  from  the 
normal  will  doubtless  cause  surprise  by  the  absence  of  periods  which  left 
an  impression  of  great  heat  or  cold.  Attention  has  already  been  called 
to  the  fact  that  an  average  temperature  for  the  period  of  a  month  or 
season  is  sometimes  an  inadequate  measure  of  the  temperature  conditions 
of  the  period.  A  month  with  10  consecutive  days  of  excessively  hot 
weather  for  example,  will  long  remain  in  memory  as  a  hot  month,  no 
matter  what  the  average  temperature  of  the  entire  month  may  be.  Yet  a 
moderately  cool  spell  preceding  and  following  the  hot  days  will  result  in 
an  average  value  for  the  month  little  if  any  above  the  normal  and  hence 
would  not  be  found  in  a  list  of  warm  months. 

The  month  of  Jul)',  1898,  may  be  cited  as  a  case  in  point.  The  highest 
temperature  recorded  in  the  official  records  of  the  Baltimore  station  oc- 
curred on  July  3, 1898,  namely  104°.  The  month  contained  10  days  with 
a  temperature  of  90°  and  over.  Yet  this  month  is  not  listed  as  a  warm 
month  because  the  average  temperature  for  the  entire  month  was  less  than 
2°  above  the  normal.  The  proper  place  to  look  for  such  excessively  hot 
spells  is  in  the  list  of  warm  days  rather  than  warm  months.  As  a  rule, 
however,  the  average  temperature  is  a  safe  guide  for  expressing  the  gen- 
eral temperature  conditions  of  a  given  period. 


MARYLAND    WEATHER    SERVICE 


97 


Warm  Months  and  Seasons. 

The  following  list  includes  the  months  and  seasons  since  1817  during 
which  the  average  temperature  rose  decidedly  above  the  normal  in  the 
vicinity  of  Baltimore.  The  degree  of  departure  required  for  each  month 
and  season  in  order  to  find  a  place  in  the  list  is  shown  in  the  column 
headed  "  Departure." 

WARM  MONTHS  AND  SEASONS. 


De- 
parture. 


January 6°+  1824,1828,1843, 

February....  6°+  1820,1827,1828, 

March 6°+  1825,1826,1812, 

April 5°+  1817,1822,1823. 

May 4°+  1822,  1820,  1833, 

June 4°  +  1828,  185S,  ISti;'), 

July 3°+  1822,  18.30.  1S31, 

August 3°+  1819,  1821,  ls:.'U'. 

September....  4°+  1822,  182t;,  istio. 

October 4°+  1855,  187'J,  issi, 

November....  4°+  1818,1822,1830, 

December 5°+  1824,  1827,  1829, 

Winter 4°  + 

Spring 3°  + 

Summer 2°  + 

Autumn 3°  + 

Year 2°  + 


1870,  1876,  1880,  1890. 
1834,  1857,  1884,  1890. 
1859,  1865,  1878,  1903. 
1827.  1865. 

1848,  1864,  1865,  1880,  1896. 
L';70. 

1.S.38,  1868,  1870,  1872. 
1S27,  1S28,  1870,  1872,  1900. 
ISSl,  ]!)00. 
lss:.>,  1SS4,  1900. 

1849,  1850,  1866,  1896,  1902. 
1848,  1857,  1877,  1881,  1889,  1891. 


1823-4,  1824-5.  1827-8,  1849-50,  1850-1,  1857-8.  1869-70,  1879-80,  1889-90. 

1822,  1826.  1827,  ls;il,  1833.  1865,  1871,  1878,  1903. 

1819,  1821,  1822,  lS-7,  182.S,  1830,  1838,  1868,  1870,  1872. 

1822,  18:30,  1854,  1.S55,  1881,  1900. 

1822,  1825,  1826,  1827,  1828,  1830,  1865,  1870. 


The  years  1822,  1827,  1828,  1857  and  1870,  are  conspicuous  in  the  list 
for  sustained  warmth  during  several  months  of  the  year.  During  1822 
there  were  five  months  of  the  year  with  an  excessive  departure  above  the 
normal.  During  one  month  only,  namely  January,  was  the  temperature 
below  the  normal.  The  year  attained  the  highest  mean  annual  temper- 
ature on  record,  namely  3.2°  above  the  normal. 

COLD  MONTHS  AND  SEASONS. 


De- 
parture. 


January 6°  + 

February  .   ...  6*-t- 

March I>°  + 

At»ril 5°-i- 

May 4°  + 

June 4°-l- 

July 3°  + 

Au(fU8t 3°  + 

September 4°  + 

OctobiT 4°  + 

November 4'+ 

December 5°  + 

Winter 4°  + 

Spriniif 3"+ 

Summer 2°  + 

Autumn 3°  + 

Year 2"+ 


1821, 
1829, 
IKlf,, 
1821, 

]K2(l, 

iKii;, 
lS2'.t. 
183t;, 
]8:t.^., 

1819, 
1820, 
1831. 


1840, 
1836, 
1843, 
18(1, 
1S3S, 
IS-Ki, 
l.SKI, 
ISlll, 
1S4(), 
1820, 
18:16, 
1840, 


1856, 
1838, 
18.56, 
1857, 
1841, 
lst;2, 

IStil. 

lS6li, 

isti:i, 
lKi4, 
1S38, 
1845, 


1857, 
18.56, 
1872, 
1S74. 
1843, 
190:t. 
1S62, 
l'.t03. 
1S71. 
IKje. 
1839, 
1872, 


1858,  1867,  1893,  1904. 

1875,  1885,  1895,  1899,  1901,  1902,  1904. 

1885. 

1861,  1882. 

1886,  1888,  1891,  1895. 


18,38,  1841,  1844,  18.59. 
1842,  1844,  1873,  1880,  1901. 
1876,  1880,  1886. 


185.5-6,  1866-7,  1892-3,  1894-5,  1901-2,  1903-4. 

1836,  1841,  184:3,  1857. 

]83t!,  1K42,  1H46,  1861.  1862,  1886,  1889,  1891,  1903. 

18:36,  1838,  1841,  1842,  1844. 

1836,  1841,  1863,  1875,  1886,  1893. 


98 


THE    CLIMATE    OF    BALTIMORE 


The  year  1836  occurs  most  frequently  in  the  list  of  cold  months  and 
seasons.  The  average  temperature  for  the  entire  year  was  the  lowest  in 
the  record  of  87  years.  It  also  contained  lowest  average  August  and  Oc- 
tober temperatures^  and  the  lowest  summer  and  autumn  averages.  Jan- 
uary alone  was  above  the  normal,  and  this  but  l.-l°  above.  The  tempera- 
ature  was  decidedly  below  the  normal  during  nine  months  of  the  year. 
The  summers  of  1903  and  1886  follow  close  behind  the  memorable  sum- 
mer of  1836.  The  recent  winter  of  1903-04:  with  an  average  temperature 
of  29.7°  was  the  coldest  experienced  in  Baltimore.  jSTo  excessively  low 
temperatures  were  recorded,  but  the  entire  season  was  characterized  by 
an  almost  unbroken  period  of  moderately  cold  weather.  There  was  an 
almost  total  absence  of  the  usual  and  sometimes  frequent  "  thaws "  of 
previous  years.  The  ice  crop  was  the  heaviest  in  many  years.  Navigation 
on  the  Bay  was  impeded  to  an  unprecedented  extent.  The  Bay  was  frozen 
from  shore  to  shore  to  a  distance  of  over  80  miles  south  of  Baltimore. 


WARMEST  AND  COLDEST  MONTHS. 
(Expressed  in  terms  of  departures  from  the  normaL) 


Normal  — 
Warmest  + 

Year 

Coldest  — . . 

Year 

Range 


Jan.  Feb.   Mar.    Apr.    May  June  July  Aug.  Sept.  Oct.    Nov.  Dec 


34.9' 

+9.0 
1858 
-10.6 
1893 
19.6 


35.8° 

+10.5 

1834 

—9.6 
1895 
20.1 


43.1 

+  7.' 
1865 
-11.9 
1843 
19.1 


54.1° 

+7.8 
1817 

-7.1 
1874 
14.9 


64.0° 

+8.3 

1826! 

-4.9 

1843: 

20.5 


73.3° 
+5.5 
1870 
—5.8 
1903 
11.3 


77.9° 
+5.61 
1870 
— 6.3| 
1891 1 
11.9! 


76.1° 
+4.9 
1831 
-5.1 
1836 
10.0 


+8.6 
1881 

-6.5 
1835 
1.5.1 


56.6°    46.6 

+7.3  +8.0 
1855  1849 
—6.7 


1836 
16.1 


1842 
14.7 


37.4* 

+8.2 
1829 
-10.4 
1831 
18.6 


WARMEST  AND  COLDEST  SEASONS  AND  YEARS. 


Winter. 

Spring. 

Summer. 

Autumn. 

Year. 

36.0° 

+7.9 

1889-90 

-6.3 

19a3-4 

14.2 

53.7° 
+5.5 
1865 
-8.9 
1843 
14.4 

75.8° 
+5.2 

1870 
-4.3 

1836 
9.4 

57.3° 

+6.0 
1881 

-4.2 
1836 
10.2 

55  6° 

+3.2 
18'^2 

Year 

Coldest— 

Year 

-3.8 
1836 

Range 

7  0 

Winter  and  spring  have  varied  most  from  the  normal,  the  difference  be- 
tween the  warmest  winter  (namely  43.9°  in  1889-90)  and  the  coldest 
winter  (29.7°  in  1903-04)  is  14.2°.  The  spring  limits  are  -f  5.5°  (1865) 
and  —8.9°   (1843),  a  range  of  14.4°.     The  summer  limits  are  +  5.2° 


MARYLAXD    WEATHER    SERVICE  99 

(1870)   and  —4.2°   (1836),  a  range  of  9.4°.     The  autumn  limits  are 
+  6.0°  (1881)  and  — 4.2°  (1836),  a  range  of  10.2°. 

A  warm  February  may  have  the  average  temperature  of  a  normal 
March,  or  approach  that  of  a  cold  April,  or  cold  October.  A  warm  Sep- 
tember may  have  the  average  temperature  of  a  normal  July  or  August. 
October  has  been  nearly  as  cold  as  a  warm  February.  May  has  been  as 
warm  as  a  cold  July.  A  month  may  have  the  same  mean  temperature  as 
the  second  preceding  or  following  month.  Hence  a  season  may  be  one 
month  later  or  earlier  than  the  average  time,  or,  there  may  be  two  months 
difference  for  example,  between  a  very  late  spring  and  a  very  early  spring. 

Frequency  of  Stated  Departures  fro:m  the  Monthly  Seasonal 
AND  AxxuAL  Mean  Temperatures. 

During  a  period  of  87  years  the  mean  annual  temperature  was  below  the 
arithmetical  average  in  45  per  cent  of  the  total  number  of  years,  above  the 
normal  in  49  per  cent,  and  exactly  normal  (within  one-tenth  of  a  degree 
Fahrenheit)  in  6  per  cent. 

the  distribution  of  seasonal  depart cres. 

Above  Normal.     Below  Normal.  Normal. 

_  <ro  %  % 

Winter 41  59  0 

Spring 48  51  1 

Summer 45  52  3 

Autumn id  54  0 

Average 45  54  ] 

These  figures  indicate  that  the  average  seasonal  temperatures  are  most 
likely  to  be  below  the  normal  value ;  hence  the  average  plus  departure  must 
be  larger  than  the  minus  departure.  This  discrepancy  between  the  depar- 
tures of  opposite  sign  is  most  conspicuous  in  the  January  temperatures 
with  departures  above  the  normal  in  41  per  cent  of  the  past  87  years,  and 
below  in  59  per  cent.  A  further  interesting  feature  of  the  tabulation  above 
is  that  the  exact  average  value  seldom  occurs.  Computing  the  averages  to 
tenths  of  a  degree  Fahrenheit,  the  normal  value  has  never  been  experienced 
in  87  years  in  winter  and  autumn,  but  once  in  spring,  and  three  times  in 
summer.  The  annual  normal  has  occurred  five  times.  Hence  the  arith- 
metical average  seasonal  temperatures  are  not  the  most  probable  or  of  the 


100 


THE   CLIMATE   OF   BALTIMORE 


most  frequent  occairrence.     The  most  probable  departure  differs  with  the 
month  and  the  season. 

The  following  table  shows  the  frequency  of  departures  of  stated 
values  from  the  normal  monthly  and  annual  temperature  during  the 
87  A-ears  from  1817  to  1903.  The  total  number  of  values  included  is  over 
1000.  For  example,  taking  the  month  of  January,  the  monthly  mean 
temperature  has  been  colder  or  warmer  than  the  normal  by  1°  or  less 
17  times  in  the  87  years  past;  it  was  3°  warmer  or  colder  16  times;  6° 
7  times;  11°  once,  etc.  The  frequency  of  stated  monthly  departures 
is  also  shown  in  Fig.  22,  and  the  seasonal  and  annual  departures  in  Fig. 
23,  as  percentages  of  total  number  of  occurrences. 


FREQUENCY  OF  DEPARTURES  FROM  NOR.MAL  MONTHLY  TEMPERATURES. 


Departures. 

t-5 

o 

< 

18 
23 
20 
10 
10 
3 
2 
3 

eS 

36 
23 
10 
6 
6 
1 
3 
1 
1 

o 

a 
a 

36 
13 
34 

7 
4 
3 

I-: 

33 

30 
11 
9 

1 
3 

1 

bi 
3 
< 

36 
25 
14 

7 
4 

1 

P. 
® 

30 

31 

17 

10 

6 

1 

1 

O 

O 

31 
18 
34 
13 
3 
4 
3 
2 
1 

5 
•^ 

20 
23 

13 
17 
8 
5 

1 
2 

o 

<B 
P 

34 
16 
11 
13 

1 

5 
3 
3 

a 
< 

45 

32 

8 

2 

±1 

17 
13 
16 
9 
12 

4 

3 

6 

.... 

11 
11 
11 
13 
11 
14 
4 
9 

31 

17 
10 
11 

10 
6 
3 

304 

3 

4 

5 

6    

180 
123 
80 
.•)5 

30 

8      .              

U 

9 

1 

11 

10 

3 

1 

1 
1 

3 

11 

1 

4 

13 

0 

+13                                                                        

1 

40 
46 

1 

1 

35 

50 

2 

47 
40 
0 

44 

41 

3 

44 
43 

0 

43 
43 

1 

39 

44 
4 

44 

42 

1 

44 
43 
0 

45 

42 

0 

43 
42 

2 

43 
43 

1 

42 
40 
5 

511 

Minus  De|>artures  ( — '' 

519 

No  Departures  (0) 

14 

In  most  months  the  departure  is  likely  to  be  about  1°  above  or  below  the 
normal;  in  April  the  most  probable  departure  is  2°,  in  October  3°,  and  in 
February  above  4°. 

Fifty-two  per  cent  of  the  mean  annual  temperatures  have  fallen  within 
1°  of  the  normal  value  in  the  past  87  years,  and  in  37  per  cent  of  the 
remaining  years  the  mean  was  within  2°  of  the  normal.  No  annual 
mean  has  risen  to  4°  above  the  normal  and  none  fallen  4°  below.  Hence 
the  annual  mean  temperature  has  a  comparatively  small  range  of  depart- 
ure from  the  normal.  The  extreme  departures  occurred  in  1822  (3.2° 
above  normal)  and  in  1836  (3.8°  below),  an  extreme  range  of  7°  between 


MARYLAXD    WEATHER    SERVICE 


101 


the  coldest  and  warmest  years  on  record  at  Baltimore.  The  frequency  of 
departures  of  stated  values  for  the  seasons  is  shown  in  the  following  table, 
in  percentages    of  the  total  occurrences  in  87  years: 


Fig.   22.  — Frequeucy  of  Stated  Departures  from  the  Monthly  Normal  Temperature. 

Fig.  22  shows  the  frequency  of  stated  departures  from  the  normal  value  of  the  monthly 
temperatures,  based  on  records  covering-  87  j-ears.  The  upper  line  of  figures  represents  the 
degree  of  departure  above  (+)  or  below  (— )  the  normal  monthly  temperature.  The  mar- 
ginal letters  represent  the  months  of  the  year.  The  curved  lines  and  shaded  areas  represent 
the  frequency  of  the  changes  expressed  as  percentages  of  total  number  of  months.  In- 
crease in  intensity  of  shading  i-eprosents  increase  in  the  frequency  of  stated  changes.  For 
e.xample,  changes  of  +  3  or  —2  occurred  in  10  per  cent,  of  the  total  number  of  instances  in 
March,  20  per  cent,  in  May  and  August,  10  per  cent,  in  December,  etc.    See  also  Fig.  23. 


FREQUENCY  OF  STATED  SEASONAL  DEPARTURES. 


Winter. . 
Spring... 
Summer. 
Autumn 


r 

2° 

3- 

4° 

5° 

6° 

7° 

8° 

9° 

% 

:% 

% 

% 

% 

% 

« 

% 

% 

19 

29 

n 

19 

10 

2 

0 

32 

27 

26 

8 

4 

2 

0 

0 

1 

46 

32 

14 

6 

1 

1 

3:3 

34 

20 

8 

4 

0 

1 

13 


winter       SprluK       Summer       Antumii     Average 


The  arithmetical  average  departure 3.4' 

The  most  frequent  departure   2.0'^ 


2.6' 
1.0' 


1.7' 
1.0' 


2.3' 
2.0' 


2.5° 
1.5" 


The  PROB.A.BLE  Error  of  the  ^Monthly  and  Annual  Means. 
Employing  the  formula  given  in  a  preceding  paragraph  for  the  deter- 
mination of  the  probable  error  of  the  daily  mean  temperature  in  order  to 


102 


THE    CLIMATE    OF   BALTIMORE 


arrive  at  the  probable  error  of  the  monthly  and  annual  means  for  the 
series  of  87  years  we  obtain  the  following  values : 

AVERAGE  MONTHLY  AND  ANNUAL  DEPARTURES  AND  PROBABLE  ERROR  OF 
THE  MONTHLY  AND  ANNUAL  MEAN  TEMPERATURES. 


Average  de- 
parture (V) 

Probable  error 
(E) 


Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

3.4° 

3.9° 

3.1" 

2.5° 

2.1° 

1.8° 

1.6° 

1.6° 

2.0° 

2.4° 

2.5° 

2.9° 

.32 

.36 

.28 

.23 

.20 

.16 

.15 

.15 

.18 

.21 

.23 

.26 

Year 


1.1° 

.10 


/ 

\ 

/ 

\ 

/ 

c^ 

\ 

/ 

'Vl 

ii. 

/ 

.' " 

v^ 

y 

/ 

\- 

\        1 

/ 

' 

I   b\    \ 

o\ 

/ 

\   \ 

k. 

\ 

/ 

\S 

^?r^ 

i^ 



/ 

SEASONAL- 
A 


ANNUAL- 

B 


Fig.  23. — Frequency  of  Stated   Departures  from    the    Normal    Seasonal    (A)    and 
Annual  (B)  Temperatures,     ia)  Summer,  (&)  Autumn,   (c)  Spring,   (d)  Winter. 

Fig.  23  shows  the  frequency  of  stated  departures  from  the  normal  seasonal  (A)  and  annual 
(B)  temperatures.  The  upper  horizontal  row  of  figures  indicates  the  degree  of  change,  and 
the  vertical  rows  to  the  right  and  left  of  the  diagi-am  indicate  the  frequency  of  change, 
expressed  as  percentages  of  the  total  number  of  seasonal  and  annual  changes  from  1817  to 
1903. 

The  probable  error  of  the  mean  annual  temperature  is  about  one-half 
that  of  the  mean  monthly  temperature ;  the  probable  error  of  the  latter  is 


MARYLAND    WEATHER    SERVICE 


103 


about  one-fourth  that  of  the  mean  daily  temperature.  Hence  the  prob- 
able error  of  the  daily  mean  is  eight  times  as  large  as  that  of  the  annual 
mean. 

SUCCESSIOX  OF  THE  SEASONS. 


It  may  be  definitely  stated,  as  the  result  of  a  study  of  the  Baltimore 
temperature  observations  for  87  years,  that  there  is  no  regular  periodic 
recurrence  of  cold  and  warm  periods.  Some  interesting  questions  in  prob- 
abilities are,  however,  suggested  by  a  classification  of  the  decidedly  cold 
and  warm  seasons,  in  connection  with  the  character  of  succeeding  seasons. 
This  has  been  done  in  the  following  manner :  A  winter  was  regarded  as 
cold  or  warm  when  the  departure  from  the  average  winter  condition 
equalled  or  exceeded  3° ;  when  less  than  2°  it  was  considered  a  normal 
season.  The  point  of  departure  for  the  spring  and  autumn  was  1.5°, 
for  the  summer  and  for  the  year  1.0°.  This  classification  yielded  from  20 
to  25  abnormal  seasons  of  each  class  during  a  period  of  87  years.  For 
each  of  the  abnormal  winters  the  character  of  the  succeeding  spring,  sum- 
mer, autumn  and  winter  was  then  noted.  Abnormal  summers  and  ab- 
normal years  were  tabulated  in  a  similar  manner,  with  the  following 
result : 

SUCCESSION  OF  THE  SEASONS. 


Cold  Winters 

(-2.0°+) 23 

Warm   Winters 

(+2.0*  +  ) 22 


Summer 

(±1.0°+). 


Autumn 

(±1.5°+). 


Winter 

(±2.0°+). 


?  be 


>  §1 


6       11 


Cold   Summers 

(-1.0°+)  ..  . 
Warm   Summers 

(+1.0°  +  )  ..  . 


Cold  Years  (—1.0°+). I    20 


Warm 


(+1.0°+).;    24 


Autumn 

(±1-5°  +  ). 


Winter 

(±2.0°  +  ). 


Spri  ng 

(±1.5°+). 


Summer 

(±1.0°  +  ;. 


1.5 


Year. 


104  THE    CLIMATE    OF    BALTIMORE 

Of  23  cold  winters,  10  were  followed  b}'  cold  springs,  13  by  average 
springs,  and  not  one  by  a  warm  spring.  Nine  were  followed  by  a  cold 
summer,  11  by  an  average  summer  and  only  3  by  a  warm  summer.  The 
succeeding  winter  Avas  cold  in  6  cases,  average  in  9  and  warm  in  8.  These 
figures  show  a  decided  probability  in  favor  of  a  cold  or  cool  spring  and 
summer  following  a  cold  winter,  and  of  a  cool  autumn;  there  is  no 
decided  tendency  as  to  the  character  of  the  succeeding  winter.  A  similar 
tendency  is  shown  in  favor  of  a  warm  spring  and  summer  after  a  warm 
winter.  There  is  also  a  decided  probability  that  a  cold  summer  will  be 
followed  by  a  cold  or  a  cool  autumn,  winter,  spring,  and  next  succeeding 
summer,  and  that  a  warm  summer  will  be  followed  by  a  warm  or  an 
average  autumn,  winter,  spring,  and  next  succeeding  summer. 

]\Iore  cautious,  and  perhaps  safer,  is  the  negative  statement  that  a  cold 
winter  is  not  likely  to  be  followed  by  a  warm  spring  or  summer;  that  a 
warm  winter  is  not  likely  to  be  followed  by  a  cold  spring  and  summer. 
In  general  it  may  be  said  that  an  extreme  season  is  not  likely  to  be 
followed  by  an  opposite  extreme.  Such  a  conclusion  may  not  be  regarded 
as  of  much  practical  value  for  determining  the  probable  character  of  a 
coming  season,  but  a  more  definite  statement  does  not  seem  to  be  war- 
ranted by  the  statistical  record. 

Considering  the  average  temperature  for  the  entire  year  there  were  20 
cases  of  a  departure  of  1°  or  more  below  the  normal.  Of  these  8  were 
followed  by  cold  years,  11  by  years  of  an  average  temperature,  and  1  by 
a  warm  year.  Of  24:  M'arm  years  3  were  followed  by  cold  years,  11  by 
average  years,  and  10  by  warm  years.  Here  again  the  same  tendency  is 
shown  against  the  occurrence  of  a  succession  of  years  of  opposite  character. 
That  is,  a  decidedly  cold  year  is  not  likely  to  be  followed  by  a  decidedly 
warm  year,  or  a  warm  year  by  a  cold  year.  All  such  classifications  are, 
however,  arbitrary  and  inferences  as  to  the  succession  of  the  seasons  should 
be  accepted  with  caution  Avhen  based  upon  phenomena  as  variable  in  their 
nature  as  the  climatic  factors  of  the  middle  latitudes. 

Daily  Exteemes  or  Temperature. 
Thus  far  only  average  temperatures  for  a  day,  month,  or  year,  have  been 
considered,  with  departures  from  the  normal  conditions  based  on  many 


MARYLAXD    WEATHER    SERVICE 


105 


years  of  observations.  The  variability  of  a  given  climate  is  best  illus- 
trated, however,  by  extremes  of  temperature  within  given  limits  of  time, 
and  by  the  frequency  of  occurrence  of  stated  changes  from  day  to  day. 

TABLE  XX.— DAILY  EXTREMES  OF  TEMPERATTRE.    (Spring.) 


Date. 


March. 


B  ® 


i-   ** ' 


April. 


O  its 


May. 


®  . 


1 72  1895  14  1884 

2 69  1882  15  1886 

:i 68  1871  12  1873 

4 74  1880  5  1873 

5 76  1880  9  1872 

6 72  1894  13  1901 

7 72  1878  12  1890 

8 a5  1878  12  1873 

9 67  1871  21  1885 

10 70  1897  17  1877 

11 71  1879  20  1892 

12 76  J890  12  1900 

13 75  1890  12  1888 

14 66  1903  14  1888 

15 '71  1886  21  1900 

16 68  1886  18  1893 

17 72  1898  15  1900 

18 68  1894  9  1877 

19 78  1894  12  1876 

20 69  1903  12  1885 

21 72  1897  12  1885 

22 82  1894  19  1885 

23 71  1871  16  1888 

24 66  1903  18  1896 

25 70  1904  21  1878 

26 66  1896  21  1878 

27 69  1903  20  1894 

28 77  18!K)  24  1894 

29 77  1902  23  1887 

30 f.9  1896  21  1887 

31 74  1888  29  1873 


78  1893  30 

82  1882  30 
80  1892  29 

83  1892  29 
77  1880  25 


75  1892  26 

75  1890  30 
85  1871  32 

82  1871  32 
8t  1887  32 

85  1887  30 

76  1899  27 

83  1890  29 

85  1896  34 
82  1891  32 

86  1896  36 
89  1896  27 
94  1896  26 
93  1896  24 

87  1896  27 


1887 
1876 
1899 
1904 
1881 

1898 
1898 
1896 
1885 
1894 

1882 
1874 
1874 
1885 
1904 

1893 
1875 
1875 
1875 
1904 


S3  1896  32  1875 

89  1902  32  1875 
88  1902  36  1875 
88  1886  38  1888 
86  1895  34  188:3 

88  1872  37  1883 

82  1891  39  1893 

81  1888  34  1898 

90  1888  33  1874 

91  1903  34  1874 


48 


87  1890  34  1876 

87  1894  37  1903 

84  1878  38  1882 

86  1892  40  1900 

84  1896  42  1875 


86  1880  40 

88  1872  44 

89  1900  42 

93  1896  40 
96  1896  42 

94  1896  45 

94  1881  44 

95  1881  40 

91  1900  46 
94  1900  42 

87  1900  43 
93  1896  44 

92  1896  46 
92  1877  47 
92  1903  48 


1891 
1882 
18ii8 
1898 
1900 

1877 
18a5 
1895 
1895 
1895 

1904 
1891 
1895 
1895 
1899 


88  1893  44  1895 

88  1903  42  1895 
90  Mt02  47  1892 

89  1884  46  1893 

90  1880  46  1877 

92  1880  4u  1886 

93  1880  48  1897 
90  1899  46  1902 
89  1895  43  1894 
95  1895  46  1884 
95  1895  50  1873 


53 
50 
46 
46 
42 

46 
44 
47 
53 
54 

49 
50 
55 
45 
52 

44 
49 
46 
45 
44 

44 
46 
43 
43 
44 

47 
45 
44 
46 
49 
45 


Table  XX  shows  the  highest  and  lowest  temperatures  recorded  on  each 
day  of  the  year  during  34  years  from  1871  to  June,  1904,  with  year  of  occur- 
rence, and  the  extreme  range  for  the  day. 


In  Table  XX,  and  in  curves  A,  C,  and  D  of  Plate  IV,  the  highest 
and  lowest  temperatures  officially  recorded  upon  each  day  of  the  year  dur- 
ing a  period  of  33  years  are  shown,  together  witli  the  absolute  daily  range. 
In  addition  the  table  shows  the  year  of  occurrence  of  the  extremes.  A 
studv  of  the  curves  of  Plate  V  will  most  clearly  and  quickly  reveal  the  ex- 
tremely changeable  character  of  the  temperature  from  day  to  day,  and  the 
8 


106 


THE    CLIMATE   OF   BALTIMORE 


relative  variability  of  the  seasonal  changes.  The  changes  in  the  average 
temperatures  from  day  to  day  throughout  the  year  have  already  been 
described  in  preceding  sections  of  this  report.  As  the  average  tempera- 
tures were  derived  from  the  daily  extremes^  there  must  of  necessity  be 
a  general  agreement,  with  a  difference  only  in  the  amplitude  of  change. 


Table  xx  Con't.— DAILY  EXTREMES  OF  TEMPERATURE.    (Summer.) 


June. 

July. 

August. 

^9 

u 

S)  be 

bi 

u 

«  be 

u 

^ 

a,  bo 

& 

a 

i-  c 

c8 

fl 

es 

u  c 

es 

c 

-^ 

.-•  7i 

93 

<s> 

« 

V 

•- 

o 

t^  cS 

O 

o 

■^  a 

Date. 

s 

>* 

g 

t>^ 

B 

S 

>< 

g 

>* 

S 

fcH 

S 

^ 

J<  u 

1 

97 

1895 

47 

1894 

50 

103 

1901 

56 

1885 

47 

95 

1890 

57 

1895 

38 

95 

1895 

48 

1897 

47 

103 

1901 

59 

1891 

44 

92 

1879 

58 

1875 

34 

3 

97 

1895 

52 

1888 

45 

104 

1898 

.59 

1888 

45 

92 

1881 

59 

1895 

33 

4 

91 

1890 

53 

1888 

38 

100 

1898 

59 

1891 

41 

94 

1888 

58 

1886 

36 

5 

93 

1899 

52 

1886 

41 

96 

1881 

58 

1892 

38 

96 

1896 

59 

1874 

37 

6 

98 
96 

1899 
1899 

47 
47 

1894 
1894 

51 

49 

96 
96 

1901 
1900 

58 
60 

1891 
1891 

38 
36 

97 
lOo 

1900 
1900 

60 
62 

1894 
1897 

37 

38 

8 

96 
98 

1874 
1874 

47 
50 

1891 
1891 

49 
48 

98 
99 

1890 
1876 

55 

56 

1891 
1891 

43 
43 

99 
100 

1900 
1900 

58 
62 

1903 
1887 

41 

38 

10 

90 

1879 

52 

1904 

38 

97 

1880 

56 

1894 

41 

100 

1900 

56 

1879 

44 

11 

92 

1893 

50 

1904 

42 

96 

1876 

57 

1898 

39 

100 

1900 

58 

1879 

42 

12 

95 

1880 

52 

1887 

43 

96 

1876 

.■)i 

1895 

39 

99 

1900 

60 

1890 

39 

13 

94 

1902 

53 

1903 

41 

99 

1880 

57 

1888 

42 

98 

1881 

.56 

1902 

42 

14 

95 

1885 

53 

1873 

42 

95 

1887 

58 

1895 

87 

96 

1872 

Oi 

1893 

39 

15 

93 

1891 

53 

1884 

40 

96 

1900 

57 

1895 

39 

90 

1900 

59 

1887 

31 

16 

94 

1891 

52 

1884 

42 

101 

1887 

59 

1903 

42 

9h 

1SS8 

58 

1889 

38 

17 

93 

1887 

55 

1899 

38 

100 

19(K) 

59 

1892 

41 

9l' 

1900 

55 

1902 

37 

18 

94 

1887 

54 

1879 

40 

102 

1887 

60 

1892 

42 

yj 

1900 

60 

1874 

31 

19 

93 

1893 

56 

1886 

37 

96 

1878 

61 

1890 

35 

9:- 

1872 

59 

1896 

34 

20 

98 

1893 

00 

1879 

43 

98 

1885 

57 

1890 

41 

97 

18i.9 

54 

1896 

43 

21 

93 

1896 

56 

1897 

37 

99 

1885 

55 

1890 

44 

97 

1899 

55 

1876 

43 

22 

94 

1888 

54 

1897 

40 

96 

1899 

56 

1890 

40 

96 

1872 

56 

1876 

40 

23 

97 

1894 

55 

1898 

42 

95 

188:^ 

59 

1890 

36 

9:- 

1898 

00 

1888 

38 

24 

98 

1894 

54 

li^02 

44 

95 

1884 

59 

1876 

;w 

94 

1898 

51 

1890 

43 

25 

98 

1898 

55 

1902 

43 

97 

1892 

59 

1876 

38 

97 

1903 

56 

1879 

41 

26 

97 

1875 

61 

1893 

36 

99 

1892 

61 

1891 

38 

96 

1900 

52 

1874 

44 

27 

95 

1876 

Oi 

1893 

38 

97 

1892 

59 

1876 

■M 

92 

1900 

53 

1885 

39 

28 

94 

1898 

56 

1897 

38 

97 

1894 

59 

1893 

38 

9J 

1895 

53 

1885 

38 

29 

97 

1874 

55 

1888 

42 

97 

1892 

62 

1897 

35 

94 

1877 

52 

1874 

42 

30 

99 

1901 

57 

1899 

42 

95 

1903 

6(1 

1880 

35 

91 

1898 

54 

1896 

37 

31 

95 

1890 

55 

1895 

40 

95 

1898 

55 

1887 

40 

The  greatest  variability  in  extreme  conditions  occurs  in  the  winter  months, 
with  a  gradual  decrease  to  more  uniform  conditions  toward  summer.  The 
greatest  change  of  temperature  which  has  been  recorded  within  a  period 
of  24  hours  during  33  years  at  Baltimore  is  47°.  This  remarkable  range 
between  the  highest  and  lowest  temperature  of  a  single  day  occurred  on 
the  24th  of  February,  1900.     When  we  consider  extremes  which  liave 


MARYLAND    WEATHER    SERVICE 


107 


occurred  upon  a  given  date,  without  reference  to  the  year  of  occurrence, 
the  range  is  greatly  increased.  For  instance,  upon  the  11th  of  February 
a  maximum  temperature  of  72°  was  recorded  in  1887,  and  a  minimum  of 
6°  below  zero  in  1899,  a  range  of  78°.    Even  in  the  months  of  least  vari- 


Table  x.v  Con't.-DAILY  EXTREMES  OF  TEMPERATURE.    (Autumn.) 


Date. 


September. 


October. 


u 

c 

X 

« 

c 

c3 

ai 

e 

S 

tH 

S 

>H 

November. 


] 95  1898 

2 96  1898 

3 97  1898 

4 i  9L  1898 

5 1  91;  1880 

6 94  1900 

7 101  1881 

8 94  1873 

9 i  94  1894 

10 1  98  1884 

I 

11 97  1897 

12 9:1  1895 

13 93  1897 

14 89  1903 

15 92j  1901 

16 89  1897 

17 90  1886 

18 91  1898 

19 94  1896 

20 5)0  1895 

21 96  1895 

22 96  1895 

23 95  1895 

24 87  1881 

25 90  1881 

26 93  1895 

27 90  1881 

28 91  1886 

2« 87  1884 

30 1  88  1881 

31 !  ! 


56  1887 

50  1892 

51  1893 
50  1872 

50  1872 

51  1883 
51  1883 
55  1892 
51  1891 
46  1883 

49  1875 

51  1879 

53  1902 

48  1902 

40  1873 


1873 

1887 
1875 
1875 
1875 


45  1897 
44  1873 

46  1896 
43  1875 

42  1887 

40  1879 

43  1879 

44  1899 
43  1903 
39  1888 


89  1881  39 

88  1881  38 

89  1879  36 
87  1884  38 
85  1884  42 


89  1884  36 

81  1884  40 

82  1887  38 

84  1893  37 

85  1887  35 


79  1898  40 
8J  1889  33 

80  1884  35 
»-2  1883  34 

83  1897  !  33 

90  1897  30 

»-Z  1879  36 

84  1881  36 
76  1899  35 

76  1884  36 

77  18.><4  39 

78  1901  34 

81  1901  36 

79  1900  34 

80  1903  33 

77  1891  30 

75  1899  35 

77  IS99  'M 

78  1874  31 
75  19113  30 
77  1896  .  31 


1899 
1899 
1899 
1888 
1901 

1893 
1893 
1876 
1896 
1895 

1881 
1876 
1876 
1875 
1876 

1876 
1893 
1876 
1880 
1900 

1900 
1899 
1889 
1889 
1879 

1879 
1903 
1898 
1873 
1873 
1893 


74  1903  31  1873 

76  1876  33  1873 

75  1903  33  I  1875 

75  1903  38  1879 

73  1896  25  1879 

74  1888  31  1892 
68  1896  28  1903 

73  1890  38  1886 

77  1895  29  ,  1886 

74  1879  31  1874 

73  1899  31  1901 

78  1879  27  1894 

76  1902  28  1873 
68  1889  I  26  ,  1873 

75  1902  28  1883 


75  ,  1897 

75  1896 

71  1896 

74  1900 

68  1900 

79  1900 

71  1883 

68  1900 

70  1896 
65  1890 

71  '  1896 
74  1896 
71  1896 

61  1879 

62  1899 


23  1883 

25  1883 

23  1891 
21  1891 
33  1879 

21  1879 

15  1880 

16  1880 

17  1880 

24  1881 

21  1903 

18  ltt03 
30  1903 
23  1875 
16  1875 


43 
44 
43 
47 

47 

43 
40 
45 
48 
43 

41 
51 

48 
42 
47 


ability  the  range  is  still  largo.  Tlio  smallest  range,  namely  31°,  is  cred- 
ited to  August  14  and  18.  The  higliest  temperature  of  the  year  occurred 
on  July  3,  1898,  and  the  lowest  on  February  10,  1899.  Although  we  have 
no  systematic  and  official  records  of  daily  cxtromos  of  tomporature  prior 
tc  1872,  when  self-registering  ma.ximum  and  minimum  tlicniidmeters 
were  added  to  tlic  (■(iiii|iiii('iit  df  the  V.  S.  AW'atlicr  liiireau  stations,  we 
have  good  reason  to  believe  that  the  period  from  1872  to  1903  comprised 


108 


THE    CLIMATE    OF    BALTIMORE 


within  its  limits  the  warmest  and  coldest  days  experienced  ar  Baltimore. 
In  the  summer  of  1898  all  existing  records  of  high  temperature  were 
broken  during  the  hot  spell  of  July  1-4  within  the  State  of  Maryland. 
In  the  following  winter  during  the  first  decade  of  February  all  existing 

Table  xx  Con't.-DAILY  EXTREMES  OF  TEMPERATURE.    (Winter.) 


Date. 


December. 


1881 
1901 

1874 
1873 
1883 

1879 
189B 
1892 
1889 
1897 

1897 
187.3 
1889 
1881 
1893 

1877 
1877 
1877 
1900 
1877 

1885 
1889 
1891 
1893 
1893 

18S9 

1881 
1889 
1893 
1898 
1-84 


1875 
1886 
1-86 
1886 
1871 


16  1901 

15  1885 

10'  1882 

4  1.S76 

1  1876 

13  1880 
22  1895 

14  1895 
14i  1898 


s  ® 

i^  C 


15 

1900 

13 

1876 

h 

1876 

12 

1875 

1(1 

18S4 

r 

1871 

5 

1871 

6 

1872 

17 

1872 

8 

1872 

8 

1872 

12 

1903 

11 

1903 

13 

1,H72 

4 

1880 

-3 

1880 

— 1 

1880 

January. 


61 


1885 
1876 
1876 
1874 
1890 

1890 

1874 
1898 
1876 
1876 

1891 
1890 
1890 
1892 
1871 

1901 

1885 
1876 
1876 
1880 

1901 

1874 
1874 
1894 
1879 

1878 
1890 
1876 
1876 
1896 
1880 


1881 
6  1899 

0  1879 

5  1879 

1877 

1904 

1884 
1878 

6  1875 
-2  1875 


1875 

1886 
1886 
1886 
1893 

1893 
1893 
1893 
1904 
1901 

1893 
1893 

1883 
1882 
1897 

1897 
1888 
1888 
1873 
1878 
1873 


13 


o  be 

n  a 


65 


65 


February. 


ii 

u 

cd 

cS 

0) 

e 

§ 

;>H 

S 

^ 

63 

1891 

8 

1900 

58 

1877 

4 

1881 

61  i  18813 
66  I  1903 


1895 

1886 


71  ,  1890  I  —1  1886 


68  <   1884 
64  :  1904 

57  :  1892 

58  1,H78 
1876 

1887 
1898 

66  1903 
63  1884 

67  j  1886 

67  1891 

73  1S91 

71  I  1891 

60  I  1887 
60  1887 

72  I  1874 

74  '  1874 


II  1895 

61  1895 

2  1S95 

2  1S99 

—7  1899 

—6  1899 

5  1899 

6  1899 
6i  1899 
6  1899 


1875 
1896 
1903 
1903 
1896 


1H74 
1872 
1871 

1890 
18>h0 
1903 

1880 


15 


1885 
1896 
1889 
1873 
1900 

1900 
1900 

1888 
1884 


C  4) 

o  be 
S  a 


records  of  great  cold  were  lowered.  The  variability  of  temperature  con- 
ditions during  the  past  five  or  six  years  has  been  phenomenal.  Eecords 
of  extremes  of  temperature  which  have  remained  undisturbed  for  many 
years  were  broken  to  a  remarkable  extent.  In  addition  to  the  instances 
of  absolute  extremes  of  temperature  just  referred  to,  may  be  mentioned 
the  high  monthly  average  temperature  of  March,  1903,  the  warmest  March 
in  87  years  or  more;  the  summer  of  1903,  the  coolest  in  87  jears  or  more; 
and  the  following  winter  of  1903-04  which  was  the  coldest  in  a  hundred 
years. 


MARYLAND    AVEATIIER    SERVICE  109 

ABSOLUTE  EXTREMES  OF  TEMPERATURE  1871-1903. 


Absolute 

Absolute 

Absolute 

Max. 

Year. 

Day. 

Min. 

Year. 

Day. 

Range. 

73" 

1889 

26 

-  3° 

1880 

30 

76° 

73 

1890 

13 

—  6 

1881 

1 

'•.9 

78 

1874 

23 

—  7 

1899 

10 

85 

82 

1894 

23 

5 

1873 

4 

77 

94 

1896 

18 

24 

1875 

19 

70 

96 

1896 

10 

34 

1876 

1 

62 

99 

1901 

30 

47 

1891 

8 

52 

104 

1898 

3 

55 

1891 

8 

49 

100 

1900 

10 

51 

1890 

24 

49 

101 

1881 

7 

39 

1888 

30 

62 

90 

1897 

16 

30 

1876 

16 

60 

79 

1900 

21 

15 

1880 

22 

64 

78 

1874 

Feb.  23 

y 

1899 

Feb.  10 

85 

96 

1896 

May  K 

5 

1873 

Mar.    i 

91 

104 

1898 

July   3 

47 

1891 

.Tune  8 

57 

101 

1881 

Sept.  7 

15 

1880 

Nov.  22 

86 

104 

1898 

July    3 

—  7 

1899 

Feb.  10 

111 

December. 
January  ... 
Febi'uary . . 


March. 
ApriL. 
May . . . 


June 

July 

August.   .. 

September. 
October — 
November. 


Winter. . . 

Spring . . . 
Summer . 
Autumn . 


Year. 


Jan               Feb             Mcm            Apr             M«»             June            Jui»            Aug             Sept           Oct             Nov             Dec              j«n 

45* 
40' 
35* 
30' 
25 
20 
I? 

4  5° 

4cr 

35° 
3(f 

25* 

20 

10' 

0* 

Fig.   24.  — Greatest  Daily   Raiitje  of   Temperature.      (See  Table  XXL) 


The  Greatest  Daily  Eange  of  Temperature, 

In  Table  XXI  the  gi-eatest  difference  between  the  daily  maximum  and 
minimum  temperatures,  or  the  greatest  daily  range,  is  entered  for  each 


110 


THE    CLIMATE   OF    BALTIMORE 


month  and  year  from  1871  to  1903,  together  with  the  daily  average  range 
for  each  ten-year  period.  There  is  a  marked  uniformity  in  the  size  of  the 
daily  ranges  throughout  the  year,  the  average  monthly  values  ranging  be- 


TABLE  XXI.-GREATEST  DAILY  RANGE  OF  TEMPERATURE. 


1871. 

1873. 
1873. 
1874. 
1875. 

1876. 

1877. 
1878. 
1879. 
1880. 


J881. 
1882. 
1883. 
1884. 
1885. 

J886. 

1887. 
1888. 

18S9. 

1890. 


1891. 
1892. 
1893. 
1894. 
189,5. 


I89f). 
1897. 
189S. 
1899. 
1900. 

1901. 
1902. 
1903. 
1904. 


20 


Means. 

1871-1880 

1881-1S90 

1891-1900 

1871-1902 


27.5 
36.3 
23.9 
35.8 


19 
28 
32 
37 
33 

29 
33 
23 

28 
24 

24 
37 
29 
26 
36 

40 
28 
.30 
35 
34 

27 
31 

36 

28 
39 

31 
22 

37 
35 
47 

33 
19 

28 
20 


37.5 
38-9 
39.3 
28.1 


29 
25 
33 
28 
30 

29 
29 

24 
28 
32 

34 

38 
36 
26 
37 

39 

38 
34 
34 
33 

34 
38 
35 
39 
30 

32 
38 
29 
32 
31 

34 
3:3 
40 
30 


20 
29 
30 
31 
31 

29 
25 

37 

a3 

30 

25 
29 
33 
35 
37 

30 
24 
24 
30 
29 

30 
30 
30 
30 
31 

39 
31 
30 

28 
35 

30 
33 

29 
33 


17 
30 
31 
38 
26 

28 
36 
23 
30 
26 

23 
25 
oo 

38 

37 

33 
29 

38 
35 


36 
37 
26 
29 

37 

24 
35 
30 

27 
31 

29 
29 
26 

37 


29.1  37. 6i  38.5  35.5 

37.4!  .30.91  37.5!  25.8 

31.1  31.8!  31.4  27.2 

29.3  30.3!  29.31  26.3 


33.4 

24.8 
26.3 
25.0 


27 
36 
36 
38 
30 

22 
36 
26 
29 


22.8 
23.5 
26.2 
24 


36 
24 

38.6 
36.5 


23 

24 
25 
29 

38 

27 
26 
25 
25 
23 

24 
23 
32 
38 
23 

28 
28 
25 
29 
21 

33 
33 

33 
30 
35 

27 
30 
28 
26 
31 

35 

27 
30 


25.5 
35.1 

30.4 

37.2 


24 

26 

24 
28 
24 

28 

27 

27 
37 
27 

81 

25 
26 

26 
27 

30 
31 

26 
24 
39 

31 

28 
28 


26.1 

36.3 

37.6 
36.8 


31 


36 
23 
42 
26 
29 

39 
23 

28 


25.4 
26.1 


26.5 


29 
35 
33 
31 
33 

38 
33 
35 
33 
33 

34 
31 
33 

30 
42 

40 
38 
34 
34 
33 

.34 
36 
35 
35 
35 


33.6 
34.8 
37.3 
35.3 


Table  XXI  shows  the  greatest  daily  range  of  temperature  (the  greatest 
■difference  between  the  maximum  and  minimum  of  any  day)  for  each 
month  and  year  from  1871  to  1904.  Also  the  average  of  the  greatest  daily 
ranges  for  the  entire  period  of  32  years  and  for  each  10-year  period. 


tween  30.3°  for  April  and  24.2°  for  August.  The  extreme  daily  ranges 
for  each  month  and  for  the  year  are  shown  in  the  following  tabular  state- 
ment and  in  Fiff.  24. 


MAEYLAXD    WEATHER   SERVICE 


111 


EXTREME  DAILY  RANGE  OF  TEMPERATURE. 


January — 
Februarj'.. 

March 

April 

May 

June 

July 

August 

September , 

October 

November.. 
December. . 


Year. 


1871-1903. 


Day. 

Year. 

Range. 

Max. 

Min. 

17 

1885 

42=' 

65 

23 

34 

1900 

47'= 

55 

8 

7 

1873 

,        36° 

50 

14 

4 

1903 

40° 

71 

31 

9 

1896 

39° 

93 

54 

11 

1900 

31° 

92 

61 

18 

1887 

1        32° 

102 

70 

6 

1900 

'       30° 

97 

67 

16 

1873 

38° 

79 

41 

19 

1901 

35° 

75 

40 

13 

1876 

31° 

68 

37 

31 

1898 

42° 

59 

17 

Feb. 

1900 

1        ''' 

55 

8 

J 

%              Fe 

e           M 

H             A 

n              M 

Af                  JU 

NE                 JU 

LV                   Al 

&.           Se 

PT 

0 

:t         Nf 

)V              D 

C.              Ja 

100° 
90° 

/ 

\ 

/ 

\ 

8(f 

^ 

/ 

y 

^ 

"\ 

\, 

^ 

r^ 

/ 

^^- 

^        "^ 

s. 

> 

7d 

/ 

y 

N. 

\ 

\, 

/ 

/ 

^^ 

^ 

\ 

s 

V 

60' 



-7 

'-/ 

Y 

^ 

\ 

N 

d 

50' 

40° 

b 

^ 

z 

7^ 

^ 

^ 

\ 

-A 
\ 

<^ 

V^ 

^ 

30' 

-r- 

^ 

7- 

:^ 

\ 

V 

20° 

-f 

^ 

10° 
0* 

/ 

J- 

\ 

• 

y 

\ 

lo' 

Fig.   '.^.5. — (a)  Extreme  .Moutlily  Ma.ximum  Temperature. 
(h)  Mean  '•  "  <i 

(c)  Average         "         Temperature. 

(d)  Mean  "         Minimum  «' 

(e)  Extreme        "  "  " 
(See  Tables  XXII,   XXIII  aud  XXIV.) 


112 


THE    CLIMATE   OF   BALTIMORE 


1871. 
18:2. 
1873. 
1874. 
1875. 

1876. 
1877. 
1878. 
1879. 
1880. 

1881. 
1882. 
1883. 
1884. 
188.5. 


1886. 
1887. 
1888. 
1889. 
1890. 

1891. 
1892. 
1893. 
1894. 
1895. 


1896. 
1897. 
1898. 
1899. 
1900. 

1901. 
1902. 
1903. 
1904. 


TABLE  XXII.-MONTHLY  MAXIMUM  TEMPERATURES. 


Xi 

o 

a 

^ 

bl 

® 

■* 

>-i 

fe 

g 

63 
56 
58 
69 
52 

71 

54 
57 
64 
65 

45 
£9 
50 
62 
65 

57 
65 
.50 
60 
73 

60 

58 
52 
57 
60 


61 
62 
78 
59 

65 
63 
63 

58 
67 

64 
59 
64 
68 
50 

67 

72 
60 

48 
74 

73 

.57 
61 
19 
62 

61 
56 
65 
60 
65 


Means. 


1871-1880 60.9  64.2 

1881-1S90 57.6  62.6 

1891-1900 58.7  61.9 

1871-1903 i  58.9|  62.3; 


69.2 

6' 

70.0 


91 


78.7 
82.8 
83.8 
82.1 


Table  XXII  contains  the  highest  recorded  temperature  during  each  month 
and  year  from  1871  to  1904,  together  with  the  average  of  the  monthly  ex- 
tremes for  the  entire  period  of  32  years  and  for  each  10-year  period.  The 
observations  were  obtained  by  means  of  a  self-registering  mercurial  ther- 
mometer, excepting  for  the  time  from  January,  1871,  to  July,  1872,  during 
which  period  the  3  p.  m.  observations  were  employed. 


Monthly  axd  Axxual  Extremes. 

The  highest  and  lowest  temperatures  recorded  during  each  month  and 
year  from  1871  to  1903  are  shown  in  Tables  XXII  and  XXIII,  together 
with  the  average  values  for  each  ten-year  period  and  for  the  entire  period. 


MARYLAND   WEATHER   SERVICE 


113 


The  absolute  monthly  extremes  of  temperature,  the  average  maximum, 
and  minimum,  and  the  monthly  normals  are  shown  in  Fig.  25.  Fig.  26 
shows  the  absolute  annual  extremes  and  the  mean  annual  temperature  for 
each  year  from  1871  to  1903. 


TABLE  XXIII.-MONTHLT  MINIMUM  TEMPERATURES. 


1871. 
1873. 
1873. 
1874. 
1875. 

1876. 
1877. 
1878. 
1879. 
1880. 

1881. 
1882. 
1883. 
1884. 
1885. 


1886. 
1887. 

lang. 

1889. 
1890. 

1891. 
1892. 
189;^. 
1894. 
1895. 


1896. 
1897. 
1898. 
1899. 
1900. 

1901. 
1902. 
1903. 
1904. 


Means. 


14 
11 

—4 
13 

—3 

17 
1 
6 
0 

17 


10 
15 

15 
i 

13 

18 
20 
12 
15 

4 
23 

23 
10 
3 

— 1 

21 

11 

3 

33 

16 
14 
11 

8 
1 

6 
18 
10 

—7 
8 


1871-1S80 7.3 

KWl-lSflO 8.H 

1S91-19UJ 11.1 

1871-1902 9.6] 


36 

9 

5 

33 

19 

13 

9 
31 

34 
23 

2T 
26 
16 
14 
12 

15 
31 
12 
38 
12 

16 
30 
16 
20 
21 

16 
28 
27 
36 
12 


14  13 

13  30 

5  ,  29 

5  I  20 


42 
38 
38 
37 
24 

30 
33 
43 
39 
30 

25 
29 
30 
34 
32 

34 
30 
33 
34 
31 

30 
32 
36 
30 
34 

31 
29 
26 
29 
30 

37 
35 
27 
27 


51 
48 
44 
41 
43 

34 
41 
43 
43 

38 

46 
38 
45 
45 
44 

45 
51 
41 
43 
43 

40 
46 
45 
43 
40 

47 
44 
40 
47 
40 

47 
43 
37 
43 


13. S  18.0  33.2  42.5 

11.9  18.3  31.2  44.1 

8.4  20.2  30.7  43.2 

ll.Oi  1».7|  38.0;  43.4! 


53.8 
r3.4 
52.5 
t3.2 


61 
55 
58 
55 


45 
50 
40 
53 
43 

45 
48 
47 
40 
50 

J9 
48 
46 
49 
46 

50 
42 
39 
46 
46 

51 
49 
44 
45 
46 

46 

45 
PS 
42 
60 

46 
48 
43 


62.4  58.6 
60.1  56.5 
57.9  57.7 
67. 6[ 


40 
38 
30 
35 
34 

30 
41 
35 
30 
35 

39 

44 
40 
35 

38 

36 
32 
36 
34 
36 

33 
34 
31 
36 
34 

36 
38 
34 
34 
36 

37 
34 
35 


28 
17 
23 
24 
16 

25 
35 
33 
30 
15 

34 

36 
33 
26 
33 

26 
25 
25 
38 
26 

18 
31 
22 
24 


15 

13 

—3 

34 
10 
17 
9 
15 

15 
16 
16 
23 
18 

17 

14 

18 


6 

—4 

13 


1 
1 
6 
0 
-3 

—6 

11 

8 
3 

— 1 

7 

9 

3 

12 

16 
12 
1 

7 
1 


■'1 


46.1  34.8'  22.5' 

47.1  37.0  26.1 

47.0!  34.6  25.0 

46.81  35.51  24.81 


11.4 
16.3 
13.8 
13.9 


2.3 
5.3 
6.1 
5.0 


Table  XXIII  contains  the  lowest  recorded  temperature  during  each  month 
and  year  from  1871  to  1904,  together  with  the  average  of  the  monthly 
extremes  for  the  entire  period  of  32  years  and  for  each  10-year  period. 
The  observations  were  obtained  by  means  of  a  self-registering  alcohol  min- 
imum thermometer,  excepting  for  the  time  from  January,  1871,  to  July,  1872, 
during  which  period  the  7  a.  m.  observations  were  employed. 


114 


THE    CLIMATE   OF    BALTIMORE 


The  absolute  range  of  temperature  at  Baltimore,  the  diflference  between 
the  highest  (104°),  and  lowest  (7°  below  zero)  recorded  readings  of  the 
thermometer,  is  111°.  The  former  occurred  on  July  3,  1898,  and  the 
latter  on  February  10,  1899.  On  both  of  these  days  records  for  extreme 
heat  and  cold  were  broken  in  many  parts  of  the  countr}'. 


871 

1875 

t880 

1885 

189C 

1895 

1900 

i9o: 

[ 

4- 

^ 

1 

: 

1 

1 

j 

] 

1 

i  1 

. 

j 

1       1 

1 

-4- 

1 

] 

-- 

1 

1 

! 

i 

V 

7/ 

^ 

^ 

-• 

i 

' 

/«! 

1 

^ 

1 A 

1 

f 

'    V 

; 

' 

. 

J 

s 

/ 

^^ 

/ 

s 

\ 

y 

s 

o^ 

/ 

V  , 

N 

/ 

vi 

/ 

**  h 

l"  >.» 

N. 

sl 

/ 

"L 

r 

j 

y 

t^ 

-- 

^ 

ft 

1 

■ 

1 

1 

I 

, 

, 

1 

-- 

1 

1 

1 

] 

! 

' 

! 

1 

I 

■ 

1 

1 

! 

i 

1 

1 

1 

' 

1 

1 

1 

1 

' 

' 

1 

' 

1 

: 

1 

1 

_• 

' 

1 

^ 

, 

' 

^ 

1 

i 

1 

! 

' 

j 

I 

1 

' 

i 

1 

1 

1 

j 

' 

1 

B 

. 

1 

1 

J 

1 

1 

__ 

^ 

^ 

=*K 

_^ 

^ 

^^ 

4^  ^ 

^tr 

J_. 

^^ 

-k. 

_L^ 

-*- 

.:^ 

-^ 

^ 

J^ 

Sr 

-_ 

__, 

[- 

-^ 

— 

~ 

-^ 

-^--L- 

-^ 

^ 

-*^ 

^ 

— 

~ 

-*- 

■  r~ 

-  , — 

-• 

1 

^ 

I 

,       \ 

' 

' 

■ 

1 

, 

1 

' 

1       [ 

1 

[ 

' 

' 

1 

1 

1 

1 

1 

1 

1 

1 

1 

' 

} 

■ 

1 

1 

' 

1 

' 

j 

' 

1 

1 

! 

1 

1 

1 

1 

' 

i 

i 

1 

' 

1 

1 

' 

t 

' 

. 

I 

1 

i 

' 

! 

1 

1 

1 

1 

j 

j 

' 

. 

1 

j 

j 

1 

1 

1 

1 

1 

1 

j 

1 

I 

1 

i  1 

t 

1 

] 

! 

1 

1 

1 

1 

j 

1 

1 

' 

j 

' 

' 

' 

1 

1 

t 

1 

1 

\ 

t 

1 

— 

-- 

- 

- 

I 

; 

1 

! 

1 

1 

! 

' 

- 

' 

1 

1 

N. 

1 

1 

, 

1 

iS 

1 

1 

1 

/-^ 

> 

1 

'T 

V 

c 

1 

' 

,r    ^ 

Jr- 

/ 

' 

r^ 

I 

/ 

' 

1 

1   / 

\ 

\ 

f 

'  , 

\ 

/ 

\ 

/ 

■ 

t 

i\ 

1 

\ 

/ 

s 

>, 

'^ 

( 

N 

J 

/ 

V 

1 

J 

'    . 

\ 

/ 

\ 

J 

' 

,' 

• 

* 

\ 

/ 

\ 

y 

- 

I 

J 

1 

\ 

- 

l—O^ 

/ 

> 

■ 

' 

J 

! 

V 

A 

^ 

, 

\ 

/ 

' 

'■ 

' 

1 

' 

j 

I 

1 

', 

1^ 

I 

' 

' 

1 

1 

^ 

] 

1 

1 

A 

I 

t 

1 

1 

t 

1 

-- 

^^ 

- 

/' 

1 

( 

' 

[ 

t 

'"I" 

1 

0 

; 

^ 

J- 

-L 

_L 

\ 

1 

_^ 

. 

_u 

Fig.  26.  — (.4)  Absolute  Annual  Maximum  Temperature. 
(J5)  Average         "  Temperature. 

(C)  Absolute        "  Minimum  Temperature. 

(See  Tables  XXII,   XXIII  and  XXIV.) 


The  Greatest  Monthly  Eange, 

The  difference  between  the  highest  and  lowest  temperatures  recorded 
during  each  month  of  the  year  is  entered  in  Table  XXIV  for  every  year 
from  1871  to  1903.  The  extreme  difference  for  each  month  during  the 
entire  period  is  shown  in  the  table  which  follows,  together  witli  the  ex- 


MARYLAXD    WEATHER    SERVICE 


115 


tremes  of  temperature  and  the  j^ear  of  occurrence.     The  extreme  range 
is  also  shown  in  Fig.  27. 

JFMAMJJASONDJ 

70' 


1 

bU 
50° 
40° 

lU 

0° 
Fig.  27.  — Greatest  Monthly   Raui^e  of  Temperature.     (See  Table  XXIV.) 

EXTREME  MONTHLY  RANGE  OF  TEMPERATURE. 

1871-1903. 


January.. 
February. 

Marcb 

April 

May. 


June 

July 

August 

September . 

October 

November. . 
December . . 


Year. 

Range. 

Max. 

Min. 

1873 

62° 

58 

—  4 

]8St) 

B8° 

67 

-  I 

18!tO 

65° 

77 

12 

19()3 

64° 

91 

27 

189.=) 

.5.5° 

95 

40 

ISitl 

51° 

98 

47 

1W8 

47° 

104 

57 

1874 

45° 

97 

52 

1873 

.53° 

93 

40 

]87!» 

59° 

89 

30 

1879 

58° 

78 

20 

1880 

59° 

56 

—  3 

Frequency  of  Days  tvith  Frost. 

As  the  frequency  of  occurrence  of  days  with  a  temperature  of  freezing, 
and  the  distribution  of  such  days  especially  in  the  autumn  and  spring 
seasons,  is  a  matter  of  greatest  practical  importance  in  agricultural  and 
commercial  affairs,  the  subject  is  here  given  more  than  ordinary  attention 
and  space.  For  purposes  of  convenience  all  days  during  which  a  minimum 
temperature  of  32°  was  recorded  are  classed  as  frost  days.  It  is  a  well  recog- 


116 


THE    CLIMATE    OF   BALTIMORE 


nizecl  fact  of  observation,  however,  that  frost  usually  occurs  before  the 
temperature  falls  to  the  freezing  point  (32°)  as  ofiBcially  recorded.  The  ap- 
parent inconsistency  is  of  course  explained  by  the  method  of  exposure  of 

TABLE  XXIV.— ABSOLUTE  MONTHLY  RANGE  OF  TEMPERATURE. 


1871. 
lS'i-2. 
1873. 
1874. 

1875. 


1876. 
1877. 
1878. 
1879. 
1880. 


1881. 
1882. 

188:^. 

1884. 
1885. 


1886. 
1887.. 
1888. . 
1889. . 
1890., 


1891. 
1892. 
1893. 
1894. 
189.5. 


1896 

1897 

1898 

1899 

1900 


1901. 
1902. 
1903. 
1904. 


Means. 


49 
4.5 
62 
56 
54 

54 
£3 
53 
61 
37 

51 
53 
39 
44 
55 

55 

58 
40 
40 
53 

39 
46 
51 
.39 
51 


1871-1880 52.3 

1881-1890  48 

1891-1900 1  47.61 

1871-1903 1  49.01 


■S     '  — 


f^   s 


56 
46 
E3 
63 
53 

53 
45 
43 
46 

63 

60 
36 
60 
58 
47 


46 
50 
51 
61 

56 
38 
55 
67 
57 

35 
46 
66 
59 


35 

43 

56 

50 

63 

37 

49 

41 

44 

50 

57 

45 

56 

48 

51 

37 

47 

54 

54 

50 

33 

59 

43 

53 

n 

44 

51.0 
53.51 
53.8 
51.71 


46 


44 

56 

45 

51 

46 

45 

63 

49 

51 

63 

53 

63 

44 

65 

.50 

55 

48 

51 

55 

54 

61 

49 

57 

54 

43 

64 

50 

53 

51.2 

45.5 

39 
41 
45 

48 
46 

54 
51 
43 
51 
55 

49 
45 
41 
44 
38 

43 
36 

46 
50 
44 

48 
41 
44 
44 
55 

49 
40 
53 
43 
54 


47 


48. 9j 
49.8! 
50.6 


51.6 
53.1 
50.3 


47.2 
43.6: 
47. 0| 
45. 7i 


37 
43 
43 
39 
88 

47 
40 
41 
51 
44 


47 
43 
43 

38 

47 
41 
35 

45 


41.4 
39.4 
43.3 
41.5 


32 
29 
34 
34 
34 

40 
39 
33 
39 
37 

31 
39 
34 
35 
43 

33 
35 
38 
33 
43 

34 
41 
38 
41 

40 

35 
33 
47 
37 
43 

39 
37 
37 
40 


34.1 
36. 3i 

38 


38 
41 
37 
45 
30 

35 
31 
33 
36 
30 

38 
33 
33 
35 
41 

34 
36 
41 

33 
44 

40 
35 
33 
36 
39 


36 
44 
53 
39 
49 

43 

40 
40 
45 
41 

42 
40 
35 
44 
40 

41 

46 
45 
38 
41 

39 
39 
44 
49 
50 


38 

41 

43 

47 

44 

43 

43 

47 

43 

50 

47 

51 

39 

43 

45 

38 

59 

58 

46 

47 

50 

47 

34 

47 

38 
54 

48 
45 

44 

48 

30 

53 

;56 

45 

39 

63 

39 

45 

.33 

46 

36 

44 

39 

46 

34 

34.6 

43.0 

36.7 

41.2 

37.1 

46.3 

36.0 

1 

43.6 

38 


53 
38 

48 
43 

53 
49 
63 
49 
40 

41 
53 
60 
44 
49 

44 
46 

48 


43 

47 
44 
49 
43 
47 

46 
49 
40 
46 
51 

46 

48 
43 
41 
51 

41 
44 

57 


44.fi  45.4  50 

44.1  45.8  45.0 

47. 9|  46.1  £0 

45.5  45.6  48 


47 
41 
43 
57 
49 

37 
43 

43 
50 
41 

50 
50 
49 
53 
47 

63 
50 
53 

68 
47 

54 
41 
41 


ei  g 
a  u 


68 
62 
50 
65 

57 
51 
53 
63 
61 


57.8 
69.0 
58.1 
58.3 


Table  XXIV  shows  the  greatest  monthly  range  of  temperature  (the 
difference  between  the  highest  and  lowest  temperature  recorded  within  the 
month)  for  each  month  and  year  from  1871  to  1904,  also  the  average  value 
of  these  monthly  ranges  for  a  period  of  32  years,  and  for  each  10-year 
period. 


the  thermometer.    Usually  thermometers  are  placed  in  a  "  shelter  "  which 
shields  the  instrument  from  undue  radiation  from  the  ground ;  the  shelter 


MARTLAXD    WEATHER    SERVICE 


117 


is  also  usually  mounted  at  a  considerable  distance  above  the  ground, 
varying  from  four  or  five  feet  to  a  hundred  feet  or  more.  In  a  quiet  atmos- 
phere with  a  clear  sky,  radiation  from  the  ground  is  very  rapid  during  the 
night  and  early  morning  hours.  The  temperature  at  the  surface  of  the 
earth  may  fall  considerably  below  that  of  the  air  but  a  few  feet  above 


TABLE  XXV.-XUMBER  OF  DAYS  WITH  A  MINIMUM  TEMPERATURE  OF  »2° 

OR  BELOW. 


Oct. 


Nov.  I   Dee.      Jan.      Feb. 


Mar. 


Apr. 


Season 


1871-3 . 
1872-3 . 
1873-4 . 
1874-5 . 
1875-6 . 


1876-7 . . 
1877-8 . . 
1878-9 . , 
1879-80  . 
1880-1 . , 


1881-  2 . 
1882-3. 
1883-4. 
1884-5. 
188.5-6. 


1886-7.. 
1887-8  . . 
1888-9  . , 
1889-90  . 
1890-1 . , 


1891-2 . . 
1892-3 . 
1893-4 . . 
1894-5 . . 
1895-6 . . 


1896-7 . 

1897-8 . 
1898-9 . 


1899-1900  . 
1900-1.... 


1901-2 . 
1902-:}  . 
190:i-4  . 


1872-1881 . 
1882-1,><91  . 
1892-1 901  . 
1872-1903 


Means. 


0.4 
0.1 
0.1 
0.2 


18 
26 
12 

20 
15 

30 
9 
30 
14 
24 

9 
15 
15 
11 
17 

25 
19 
16 
8 
21 

10 
17 
20 
15 
14 

20 
13 
90 
19 
19 

19 
23 
25 


7.0  I  18.8 
4.8  i  15.6 
5.5  i  16.9 

6.1  I  17.5 


19 
31 
14 
29 
17 

29 
19 
27 
9 
36 

19 

35 
34 
20 
24 

24 
29 
16 
10 
18 

33 

28 
17 
26 
22 

32 
15 
21 
20 
23 


21.0 
20.9 
21.7 
21.6 


17 
18 
19 
24 
19 

19 
14 
25 
17 
30 

12 

18 
11 
34 


19.3 
16.6 
19.9 

18.8 


13.1 
13.8 
11.2 
13.3 


2.3 
1.3 
1.8 
1.9 


73 

94 
75 
104 

77 

103 
48 
85 
64 


61 
80 
78 
81 

87 
64 
114 


81.8 
73.1 
77.1 

78.4 


under  such  conditions.  Thus  we  may  have  frost,  especially  in  the  low 
places,  when  the  thermometer  records  a  minimum  temperature  of  35° 
or  40°,  according  to  the  position  of  the  instrument.  In  view  of  these 
facts  the  figures  entered  in  the  following  tables  to  represent  the  fre- 
quency of  frost  days  must  be  regarded  as  the  lower  limit  of  frequency ;  the 


118 


THE    CLIMATE   OF   BALTIMORB 


figures  would  be  increased  to  a  small  extent  by  placing  the  thermometer 
nearer  the  ground.  To  enumerate  frost  days  upon  the  basis  of  the  actual 
observation  of  frost  on  the  ground,  introduces  additional  diflficulties  as 
the  production  of  frost  depends  not  only  on  a  temperature  of  32°  or 


TABLE  XXVL- 


-LONGEST  PERIOD  OF  CONSECUTIVE    DAYS    WITH  A  MINIMUM 
TEMPERATURE  OF  33°  OR  BELOW. 

D^'ysf  '^*™''  °*  Occurrence. 


1871-3. 
1873-3. 

1873-4. 
1874-5. 

1875-6. 


1876-7.. 
1877-8.. 
1878-9  . 
1879-80. 
1880-1.. 


1881-3. . 
1883-3.. 
1883-4.. 
1884-5. . 
1885-6. . 

1886-7.. 
1887-8.. 
1888-9.. 
1889-90. 
1890-1.. 


1891-3. 
1893-3. 
1893-4 
1894-5. 
1895-6. 


1896-7.... 

1897-8 

1898-9.... 
.1899-1900. 
1900-1.... 


1901-3 . 
1903-3. 
1903-4. 


Means. 

1873-1881. 

1883-1891. 
1893-1901. 
1873-1904. 


15 
33 
14 

30 


37 
9 

10 
9 

13 

24 

9 

34 

15 

15 

15 
34 
13 

38 


Jan.  33-Feb.  5 
Dec.  9-Dec.  31 
Jan.  30- Feb.  13 
Dec.  30- Jan.  38 
(  Dec.  13-Dec.  31 
(Jan.  30-Feb.  7 


Dec. 
Jan. 
Dec. 
Feb. 
Jan. 

Dec. 
Jan. 
Jan. 
Jan. 
Jan. 

Dec, 
Jan. 

Feb. 
Mar. 
Feb. 

Mar. 
Jan. 
Dec. 
Jan. 
Feb. 

Jan. 
Jan. 
Jan. 
Dec. 
Jan. 

Jan. 
Feb. 
Dec. 


1.5-Jan.  18 
38-Feb.  7 
16-Jan.  24 
1-Feb.  11 
23-Feb.  8 

30-Jan.  7 
2-Jan.  17 
3-Jan.  10 

10- Jan.  27 
6-Jan.  26 

3.5-Jan.  30 
9-Feb.  4 

19-Feb.  37 
l-Mar.  10 

37-Mar.  7 

11-Mar.  32 
3- J  an.  36 
1-Dec.  9 

23-Feb.  25 
9-Feb.  23 

33-Feb.  6 
27- Feb.  10 
35-Feb.  17 
3.5-Jan.  5 
23-Mar.  1 

26-Feb.  31 
16-Feb.  37 
36-Jan.  31 


under  at  the  place  of  formation,  but  also  upon  the  relative  amount  of 
moisture  in  the  atmosphere.  The  actual  observation  of  frost  has,  however, 
been  employed  in  the  table  in  which  first  and  last  frosts  of  the  autumn 
and  spring  respectively  are  recorded  and  a  minimum  temperature  of  32° 
resorted  to  only  in  case  of  conditions  unfavorable  to  the  occurrence  of 
frosts  on  account  of  a  dry  atmosphere. 


MARYLAND    WEATHER    SERVICE 


11& 


In  making  a  comparison  of  the  relative  severity  of  winter  seasons  the 
frequency  of  occurrence  of  a  minimum  temperature  of  32°  or  under  is  in 
some  respects  a  more  satisfactory  test  of  the  general  character  of  the 
season  than  the  usual  one  of  the  average  temperature. 

In  Table  XXV,  the  frequency  of  occurrence  of  such  days  is  shown  for 
each  month  from  October  to  May,  and  the  total  number  for  each  year  from 
1871  to  1904.  The  season  having  the  greatest  number  of  frost  days  from 
1871  to  1904,  is  that  of  1903-04  when  114  were  recorded.  There  were  104 
in  1874-75,  and  102  in  1876-77.  In  1877-78  there  were  but  48 ;  in  1889- 
90,  49.     The  average  number  for  the  entire  period  of  33  3-ears  is  78. 


LLJ I  I  I  I I  I  I  I — LI — I— 


Fig.  :i8.  — Longest  Period  of  Cousecutive  Days  with  a  >[inimura  Temperature  of 
32°  or  Below.     (See  Table  XXVI.) 


In  Table  XXVI  the  longest  period  of  consecutive  days  with  a  minimum 
temperature  of  32°  is  recorded  for  each  year,  with  the  time  of  occurrence; 
the  duration  of  these  periods  is  also  presented  graphically  in  Fig.  28. 
The  cold  periods  of  this  class  begin  most  frequently  in  the  month  of 
January,  though  many  begin  in  December.  In  one  instance  the  longest 
uninterrupted  cold  spell  fell  entirely  within  the  month  of  March,  namely 
in  1890,  from  March  1-10. 

The  season  credited  with  the  longest  period  of  consecutive  days  witli  a 
minimum  of  32°  is  that  of  1878-79  when  the  minimum  was  32°  or  below 
daily  without  interruption  from  December  IG  to  January  24,  or  40  days. 
In  the  winters  of  1875-7G,  1881-82,  1883-84,  1888-89,  1890-91  and  1893- 
94.  the  longest  period  was  9  days.  The  average  lengtli  of  uiiinti'rru])t('d 
periods  of  freezing  weather  is  19  days. 


130 


THE    CLIMATE   OF   BALTIMORE 


The  cold  days  occurring  in  Baltimore  from  1871  to  1904  were  also 
tabulated  on  the  basis  of  a  daily  mean  temperature  below  32°  and  below 
14°.  The  results  are  shown  in  Fig.  29  for  the  entire  season.  The  aver- 
age winter  season  contains  33  days  with  a  mean  temperature  below  freez- 
ing point.  The  winter  season  of  1903-04  contained  66,  the  highest  num- 
ber recorded  during  any  year  of  the  period;  the  seasons  of  1884-85  and 
1901-02  follow  with  50  each.     In  1877-78  there  were  but  17;  in  1879-80 


1875-6 

1880-1 

1 885-6 

1890-1 

1895-6 

1900-1 

DAYS 

A 

60 
40 

|0 

B 

0 

.J 

L 

_l 

L_ 

_l 

U 

_l 

II 

_l 

L 

J 

1 

J 

L 

-J 

L. 

Fig.  29.  — (A)  Number  of  Days  with  Meau  Temperature  below  32° 

(j5)         a  a  .1  u  ..         140 

and  1881-82,  13;  and  in  the  winter  of  1889-90,  but  10,  the  lowest  number 
on  record.  The  average  number  for  each  10-year  period  from  1871-1904 
is  shown  in  the  following  table: 

AVERAGE  NUMBER  OF  DAYS  WITH  A  MEAN  TEMPERATURE  BELOW  32° 


1871-1880. . 
1881-1890. . 
1891-1900. . 

Decade. 

Nov. 

0.8 
1.3 
1.3 

Dec. 

8.4 
6.9 
7.6 

Jan. 

9.8 
12.. 5 
11.8 

Feb. 

8.8 
7.8 
9.1 

March. 

2.9 
4.8 
3.7 

April. 

0.1 
0.0 
0.0 

Season. 

30.8 
33.3 
33.5 

1871-1904. . 

1.3 

8.0 

11.9 

9.5 

3.6 

0.0 

34.3 

Greatest  number  (1903-4) . . . . 
Least  number  (1889-90) 

6 

0 

15 
1 

23 
2 

20 
1 

2 
6 

0 
0 

66 
10 

MARYLAND    WEATHER    SERVICE 


121 


The  frequency  of  days  during  which  the  highest  temperature  of  the  day 
fell  below  the  freezing  point  is  shown  in  Fig.  30.  The  average  monthly 
and  annual  frequency  for  the  entire  period  of  33  years  and  for  each  decade 
is  given  below. 


AVERAGE  FREQUENCY  OF   DAYS  WITH  A  MAXIMUM  TEMPERATURE 

BELOW  32* 


Decade.  Nov.  Dec.  Jan. 

1871-1880 0.3  3.;  4.7 

1881-1890 0.3  2.3  6.7 

1891-1900 0.1  3.5  6.3 

1871-1904 0.2  3.6  6.0 

Greatest  number  (1892-3) 0  10  19 

Least  number  11877-8)    0  0  3 


Feb.      March.    Season. 


5.9 


0.8 

12.0 

1.6 

13.6 

0.9 

16.7 

4.3 


1.0 


15.1 


87>« 

880-1 

88 

D-D 

890-1 

89>6 

900-1 

J 

J 

Fig.   30. — Annual  Frequency  of  Days  with  a  Maximum  Temperature  below  32°. 


With  an  average  annual  frequency  of  15  days,  the  number  varied  from 
3  in  1877-78  to  36  in  1892-93.    The  winter  of  1903-0-1  contained  21. 

In  Fig.  31,  the  annual  frequency  of  cold  days  is  shown  on  a  basis  of  the 
occurrence  of  a  daily  minimum  temperature  of  20°  in  the  months  of  De- 
cember, January  and  February  and  a  minimum  of  28°  in  November, 
March,  and  April.  In  the  table  below  the  average  monthly  and  seasonal 
frequency  of  occurrence  of  such  days  is  indicated  for  each  ten-year  period 
from  1871  to  1904,  and  for  the  entire  period  of  33  years. 


123 


THE    CLIMATE    OF    BALTIMORE 


187 

« 

188 

OJ 

188 

5-6 

1890-1 

1895-6 

1900-1 

DAYS 
50 

-J 

40 

-J 

30 

1 

- 

10 

- 

0 

_ 

Fig.   31.  — Annual   Frequency  of  Cold  Days. 

20°  or  less  in  December,  January  and  February. 
28°       "       "      November,  March  and  April. 

(See  Tables  XXVII   and  XXVIII.) 


187 

5-6 

18801 

1885-6 

1890-1 

1895-6 

1900-1 

1 

1 

1 

1 

-Dec. 
1 

1 

1 

Feb 

i 

1 

1 

1 

1 

1 

1 

1 

1 — 

-  Apr 

1   1 

1 

1 

l_ 

—i 

I— 

_l 

1_ 

Fig.    32. — Monthly   Frequency   of  Cold  Days. 

(a)  20°  or  less  in  December,  Jauuary  and  February. 
(&)  28°       "       "      November,   March  and  April. 

(See  Tables  XXVII  and  XXVIII.) 


MARYLAND    WEATHER    SERVICE 


133 


FREQUENCY   OF    DAY^S    WITH  A  MINIMUM   TEMPERATURE  OF  20°  IN  WINTER 

AND  28°  IN  SPRING  AND  AUTUMN. 

Decade.                         Nov.  Dec.  Jan.         Feb.       March.  April.  Season. 

1871-1880 2.8  5.0  0.0             5.2            6.1  0.5  24.8 

1881-1890 3.0  4.1  8.2             4.7             6.9  0.1  27.0 

1891-1900 2.3  4.2  7.1              6.6             6.6  0.1  26.9 

1871-1904 2.8  4.6  7.3            6.2            6.0  0.2  27.1 

Greatest  number  (1903-4) 11  7  14              18               4  1  55 

Least  number  (1881-2) 3  0  7               0               1  0  11 


TABLE  XXVIT.-LIST  OF  COLD  DAY'S.-January. 

(Minimum  of  20°  or  below.) 

DAY   OF  MONTH. 


Year. 

1 
o 

2 
o 

3 
o 

i 
o 

6 
0 

6 
0 

7 
0 

8 
0 

9 

0 
19 

1011 

1 

12 
0 

13 

0 

14 

0 

15 

0 
20 

ii 

16 
12 

is 
9 

16 
0 

is 

20 
26 

17 

0 

i3 

ie 
26 

18 
0 

ii 

26 
i2 

19 
0 

26 

202 
0 

ii.' 

i8i 

20. 
26  i 

122 

0  0 

23 

0 
14 

24 

_ 

0 
17 

25 
0 

i9 

362 

0 
15. 

7282 
0  0 

«  30 
0  0 

31 
0 

g 

Is71 

0 

17 

0 

>i 

4  18 
6—4 

11 

8 

4 

3 

14 

1 

4 

18. 

*) 

,5 

15 

18 

6 

2 

2 

17 
17 

19 

20 

i7 

20 
6 

H 

fi 

h'.'. 
!i9 

41<s 
.10 

6 '9 

ii 
11 

i.5 

17 

ii 

i9 

18 

'i 

IS 
12 

is 

ie! 

;? 

10 

13 

18 
0 

6 
'5 

1 

20 
6 

10 
15 
14 

ii 

14 

6 

11 

13 
26 

V 

K 

9  19 
0  .. 

H 

9 

6 

ii 

7152 

q 

1,S,<| 

1 

1 

—6 

16 
19 

9 
18 

i7 

i9 

i7 

20 

15 

i7 

6 
12 
17 

20 

is 

4 
i4 

,', 

10 

7 

ii 

s 

4 

12 

8 

9 

20 

11 
14 

;io 

18 

i5 

16. 

V' 

.5 

18 

0141 

1  18 

s 

'9 
19 

iei 

20. 

11 

12 

7 

13 

__ 

20 

13 

26  i 

sioi 

10 

s 

11 

y 

1 

1M90 

.20 

ii9 

3  8 

16 

ii 

191 

is! 

i!! ! 

1 

1 

i2 
12 

20 

'2 

'" 

26 
11 

ie 
26 

16 

ii 

isi 

161 
26' 

so! 

14 

0 

8 

3 

11 

17 
9 

19 
18 

16 

6 

13 

26 

is 

Ifl 

4 

17 

i9 

1 

.1 

i  is 

•i  19 

)  i-i 

20 

ii 

26 
10 

19 
20 

8 

i; 

'si 
m 

ejisi 

0182 
9..1 

4 

10 

H 

9 

1900 

1 

9 
15 

18 
6 
14 

it> 

20 

18 
19 

is 

20 
20 

ii 

iti 

19 

1 

'9 

is 

2 

12 

6 

io 

26 

201 

■  19 

7 

3 

9 

4 

17 

8 

8 

2 

5 

.. 

6l.^l. 

..  ..1 

■17. 

18 

20 

14 
•49 

1 

1 

1 

Table  XXVII  contains  a  complete  list  of  all  days  from  1871  to  1904  during 
which  the  temperature  fell  to  28°  or  lower  in  November,  March  and  April, 
and  to  20"  in  December,  January  and  February;  together  with  the  minimum 
temperature  recorded  on  the  corresponding  days,  and  the  number  of  such 
days  in  each  month.  Fig.  31  shows  the  annual  frequency  of  cold  days,  and 
Fig.  32  the  monthly  frequency. 


124 


THE    CLIMATE    OF    BALTOIORE 


Cold  days  of  the  class  described  in  the  above  table  occur  most  fre- 
quently in  the  month  of  January,  as  is  the  case  with  the  other  classes 
tabulated.    A  peculiarity  in  the  seasonal  distribution  is  however  revealed 


TABLE  XXVir.-LI8T  OK  COLD  DA YS.-Februaiiy. 
(Minimum  of  20°  or  below. i 

DAY  OF  THE   MONTH. 


1 
0 

3 

0 

3 

4 
0 

5 

0 
16 

is 

16 

6 

0 
10 

26 
18 

7 
0 

i2 

8 
0 

io 

9 
0 

ig 

15 
6 

10 
0 

is 

'4 

11 
0 

12 
0 

is 

13 
0 

14 
0 

is 

.. 
20 

is 
26 

is 

IS 

0 

17 
15 

io 

.. 

26 

18 

16 
0 

'6 
26 

0 

i2 

io 

•  • 

ii 

IS 
0 

■9 
26 

ig 

8 

i3 

13 

19 
0 

is 

ii 

ii- 
ig 

ii 

is 
5 

20 
0 

26 

io 
ig 

ii 

8 

is 

is 
12 

16 

21 
0 

ig 

"s 
20 

12 
11 

ig 

19 

22 
0 

ii 

18 

23 
0 

i2 

13 

is 
i2 

16 

24 
0 
'2 

12 
14 

'3 

is 

20 

'8 
16 

26 
0 
30 

" 

ii 

"s 

*8 
19 

ig 

26 
0 

ig 
26 

is 
is 

37 
0 

17 

ig 

'g 

.. 

28 
0 

is 
ii 

ie 

18 

i.3 
is 

2g 
0 

io 

3 

0 

H 

1871 

3 

n 

16 

17 
18 
20 

16 

13 

ig 

4 

3  

H 

4 

0 

5 

20 

17 

6 

6 
1 

9  ....  ..........    [.........[ 

20 

20 

.. 

-• 

3 
5 

1880 

15 
4 

'' 

12 

8 

19 
14 

-1 

18 
7 

15 
26 

19 

17 

is 
12 

'3 

•  • 
u 

13 

14 
26 

4 

1 

o 

3'.'.'.'.'..'.'.'.'.'..'.'.'.'.'.'.'.'.  ".'.'.'.'. 
i 

5 

15 

8 
0 
0 
2 

1^ 

6 

18 

11 

8 

ii>'.'. 

0 
6 

9 

18 

is 
i 

17 

18 
0 

'4 

8 

1890 

1 

20 

18 

u 

26 
is 

0 
9 

4 

3 

16 
14 
9 

[on 

•S 

4 

19 

20 
15 

5 

ig 
26 

i2 

6 

4 

17 

r 

16 

T) 

6 

18 
14 

8 
8 

14 

io 

14 

8 

19 

i2 
is 

ii 
is 

19 
13 

IS 
14 

16 

8 
ii 

19 

ig 

-6 

ig 

*5 
19 

6 

16 
18 

'e 

14 
16 

6 

1 

8 

Ii 

9 

11 

1900 

11 

1 

11 

1' 

3 

5 

4 

12 

11 

16 

17 

19 

..18 

14 

20 

IS  14 

18 

17 

18 
211 

in  the  comparatively  high  frequency  of  occurrence  of  such  days  in  March, 
after  making  due  allowance  for  the  fact  that  the  March  minimum  is 
8°  higher  than  the  minimum  for  the  winter  months. 

The  distribution  of  cold  days  of  this  class  by  months  and  years  is 
shown  in  Fig.  32;  a  complete  list  with  temperatures  recorded  is  con- 
tained in  Table  XXVII. 


MARYLAND    WEATHER    SERVICE 


125 


TABLE  XXVIL— LIST  OF  COLD  DAYS.— March. 
(Minimum  of  28°  or  below.) 

DAY   OF  MONTH. 


Year. 

o 
27 

2 

o 

24 
27 

8 

i 

5 

6 

7 

o 

23 

14 

8 

9 
o 

10 

0 

11 
o 

12 
o 

38 

13 
o 

23 

14 

0 

35 

,. 

3« 

15 
o 

24 

16 

o 
37 

27 

IT 

o 

25 
20 

13 

0 

23 

is 

9 

19 

0 

12 
15 

20 
o 

ig 

24 
12 

21 

o 
2i 

26 

26 
27 

32 
o 

i9 
25 

23 

0 

24 

0 

23 

.. 

36 

23 

i9 

38 
37 

18 

25 

0 

28 

2i 

22 
36 

25 

26 

0 

2i 

28 

.. 
37 

27 
o 
23 

30 

37 

28 

0 

28 
24 

29 
o 

23 
24 

38 

30 

0 

3i 

28 

•• 

31 
o 

o 
H 

1871 

o 

24 

12 

o  o 
is  9 

0 
10 
9 

1 

8 
12 
3 
2 

4 

4 

1 
11 

3 

4  

51013 

5 

fi 

20 

20 

27 

37 

26 

28 

36 

i? 

21 

g 

9 

l^^iO 

24 

36.. 

28 

28 

ii 

25 
20 

24 

25 
12 

16 

26 

ii 

3^ 

24 

18 

24 

21 

25 

28 

25 
25 

23 

33 

23 

18 

28 

ii: 

37 

38 

24 
24 

27 

25 

27 
28 

ih 

28 

io 

26 
24 

24 

25 

27 

14 
23 

20 
23 

■■ 
20 

12 
26 

22 

i2 

3i 

3i 

33 

36 

38 

24 
i9 

ie 

23 
■»7 

26 

is 

24 
26 

1 

^ 

27 

27 

1 
..:28 

28  i9 
1823 

28 
27 

22 

ie 

35 

2i 

37 

2.5 
20 

4 

14 

24 

23 

S 
13 

3 

9 
13 

1 
10 

9 
11 

i 

5 

6 

S. ...'.'..'.....      '".'.'.'.'.'.'.'. 

9 

1890 

1 

•> 

s'.'.'.'.'.'.'.'.'.'.".'.'.'.'.'.'.'.'..'.'.'. 

i 

.5 

19 

26 

20 

15 

24 

16 

23 

19 

24 

28 

27 
24 

34 

25 

20 

24 
16 
21 

9i 

is 

32 
34 

20 
i2 
27 

24 

1>3 

15 

\ 

9 

27 

__ 

..'.. 

2i; 

26 

28 

22 

is 

36 

21 

32 

1 
4 

1900 

1 

q 

28 

..19 

13 
36 

14 

<| 

3 

3 

4 

'■ 

'.'ki 

j6 

0 
4 

307 

TAI5LE  XXVII.-LL><T  OF  (OLD   DAYS.-Apkil. 
(Minimum  of  28°  or  below.) 

DAY  OF  THE  MONTH. 


Year. 

Iy74 

1 

0 

0 

3 

0 

4 

0 

5 

0 

38 

6 

0 

36 

7 
o 

.. 

8 

0 

9 
o 

10 

0 

11 

0 

12 

0 

37 

13 
o 

H 
o 

15 

0 

16 
o 

_ 
0 

1. 

26 

19 

0 

24 

20 
o 

37 

21 

0 

22 

0 

a' 

0 

24 

0 

.. 

0 

26 

0 

37 
o 

38 

0 

2930 
0  o 

1 
o 

]ij7S 

8 

ly((l    .     

26 

1 

1 

1903           

27 

1 

1904 

1 

9 

126 


THE    CLIMATE    OF    BALTIMORE 


The  Fkequexct  of  Cold  Waves. 

A  cold  wave,  to  come  within  the  definition  of  the  U.  S.  Weather 
Bureau,  is  a  fall  in  temperature,  in  the  horizon  of  Baltimore,  of  20° 
within  twenty-four  hours,  to  a  minimum  of  20°  in  December,  January, 


TAHLE  XXVIL— LIST  OF  COLD  DAYS— Novem her. 
(Minimum  of  28°  or  below.) 

DAY  OF  THE  MONTH. 


Year. 

1 

0 

■■ 

o 

0 

3 

0 

4 
o 

5 

0 

6 
o 

7 
o 

8 

0 

9 
o 

10 
o 

11 

o 

12 
o 

13 

o 

14 

0 

15 
o 

16 

o 

17 

0 

28 

18 

0 

27 

19 

o 

38 

23 

20 
o 
24 

33 

31 

0 

26 
32 

2i 

28 

32 
o 

20 
15 

2334 

25 

26 

27 

28 

29 

30 
o 

ii 

25 
24 
16 

35 
35 

28 

37 

38 

18 
37 
25 

1 
o 

3871 

o 

28 

ie 

0 

ii 

28 

0 

,', 

24 

.. 

:'' 

28 
35 

36 

36 

o 

35 

28 

28 

33 
26 
2i 

0 

22 

26 

24 
34 

is 

o 

28 
37 

26 

37 

35 
20 

o 
38 
20 

23 

26 

25 

27 
24 

24 

34 
24 

1 

5 

3 

28 

26 
27 

28 

7 

4 

9 

5 

3 

fi 

.. 

28 

S5 

1 

8 

!( 

1 
0 

1880 

s 

1 

3 

38 

i\ 

3 

38 

33 

25 

38 

4 

4 

1 

5 

(i 

38 

35 
26 

J5 

.. 

25 
36 

21 

0 
3 

3 

}< 

3 

9 

1890 

1 

22 

21 

1 
3 

4 

o 

3 

S    

28 
26 

28 

4 

4 

3" 

26 

;i 

5 

f, 

6 

8 

0 

1 
4 

9 

1900 

1 

28 

37 
25 

25 

28 

0 

1 

4 

3  ........  .........[[.[.[[.[ 

38 

■• 

•'fi 

'>\ 

0 
11 

94 

and  February,  and  to  a  minimum  of  28°  in  ]March  and  November. 
The  designated  minimum  must  be  reached  not  later  than  12  hours  after 
the  expiration  of  the  24-hour  period.  Thus  three  events  are  essential 
for  the  technical  verification  of  a  cold  wave,  namely:  (a)  a  fall  of  20°; 
(b)  the  fall  must  occur  within  a  period  of  24  hours;  (c)  a  designated 
minimum  temperature  must  be  reached  witliin  36  hours.     Cold  waves 


MARYLAND    WEATHER    SERVICE 


127 


fulfilling  all  these  conditions  are  not  of  frequent  occurrence  within  the 
geographical  horizon  of  Baltimore.  It  has  been  shown  in  the  para- 
graphs dealing  with  diurnal  variability  of  temperature  to  what  extent 
the  frequency  of  occurrence  of  given  changes  in  temperature  from  day 


TARLE  XXVII.-LIST  OF  COLD  DAYS.— December. 

(Minimum  of  20"^  or  below.) 

DAY  OF  THE  MONTH. 


Year. 

1 

o 

2 
o 

3 

0 

4 
o 

5 

0 

12 

6 

7 

8 

' 

10 

11 

12 

« 

14 

15 

16 

17 

18 

19 

« 

21 

22 

23 

24 

25 

26 

27 

2829 

30 

31 

■5 
1 

11*71 

o 
17 

0 

0 

is 
in 

0 

i 

0 

is 

ii 

0 

20 

18 

is 

o 

o 

o 

o 

ii 

o 
13 

0 

ie 

8 

o 

i2 
15 

ie 

17 

o 

ie 

10 

io 

0 

7 

i2 

12 

9 
20 

0 

5 
14 

ie 

20 
20 

14 

o 
16 
6 

11 

o 
i7 

is 
is 

is 

20 

0 

's 

20 

ii 
is 

14 
16 

0 

's 

18 

is 

■■ 

•  ■ 
■■ 

i4 

18 
16 

ig 

o 

ii 

•• 

ie 

IS 
20 

20 

ii 

20 

20 

is 

12 

o 

ii 

is 
io 

20 

ii 

is 

20 

11 

o 

is 

18 
20 

ii 

is 
ii 

15 

is 
is 

19 

0 

ie 

•  • 

4 

ie 
is 

is 

'7 

20 
ie 

i9 

0 
,', 

-3 
20 

17 

ig 

is 

16 

ii 

0 
17 

-i 

26 
26 

17 

ii 
9 

5 

10 
0 

1 
5 

le 
0 

6 

9 

9                   .... 

17 

ie 

18 

20 

.. 

3  

4 

5 

fi 

T 

8 

9 

1880  

1 

1=) 

ii'C. 

0 
3 

3 

4 

3 

6 

6 

l*)!"! 

IS 
.. 
19 

15 

16 

<> 

6 

17 

16 

18 

18 

20 

11 

8 

18 

4 

9 

1890 

20 

0 
5 

1 

1 

o 

10 

3  

18 

•• 

i4 

is 

20 

is 

i4 
i9 

is 
is 

19 

19 
17 

ig 

19 
18 
20 

20 

17 

1R 

0 

4 

3 

6 

19 

7 

8  

2 

1 

9 

1900 

1 

20 

16 

18 

6 
4 

11 

§!!!!!!!. .............. 

'w" 

•■  •■  i 

154 

to  day  depends  upon  the  method  of  determining  the  change.  Basing 
the  20°  fall  upon  the  minimum  temperatures,  the  8  a.  m.  or  8  p.  m. 
temperatures  from  day  to  day,  the  Baltimore  records  from  1871  to  190-1 
show  a  frequency  of  cold  waves  indicated  in  tlie  table  below.  The  table 
shows  the  extent  of  tlie  fall,  and  the  minimum  temperature  attained 
within  ."50  hours. 


128 


THE    CLIMATE   OF    BALTIMORE 
TABLE  XXAail.-FREQUENCY  OF  COLD  WAVES. 


1870-1 . 


1871-2. 


1872-3.. 
1873-4.. 
1874-5. . 

1875-6.. 

1876-7.. 

1877-8.. 
1878-9.. 
1879-80. 


1880-1. 
1881-2. 


1882-3. 


1883-4.. 
1884-5. . 


1885-6.. 

1886-7  . 

1887-8.. 
1888-9.. 
1889-90. 


1890-1. 
1891-2. 

1892-3. 

1893-4. 
1894-5. 


1898-9. 


1899-1900. 


1900-1. 
1901-2. 
1902-3 


Nov. 


Fall  I  Min. 


0 
36 

0 

0 

26 
0 
0 

0 

35 
22 


1895-6 21 

1896-7 0 

1897-8 0 


1903-4  21 


Total  number 8 

Average  number. . .     0.2 


Dec. 


Fall   Min 


19 
0.6 


13 

7 

20 

8 

0 

19 

15 

9 

13 
0 
0 

18 

0 
0 

12 

0 
10 

0 

IS 

15 

16 

0 

0 

0 
0 

0 
0 


Jan. 


Fall   Min 


20 


23 
0.7 


Feb. 

March. 

Fall 

Min. 

Fall 

Min. 

Q 

0 

^ 

^ 

21 
21 

15 

20 

0 

0 

26 

15 

21 

9 

19 

27 
0 
24 

20 
0 
16 

21 

0 
25 

7 

0 

26 

20 
25 

17 
16 

0 

0 

0 

0 

33 

23 

0 
0 
0 

0 
0 
0 

27 
0 
0 

25 
0 
0 

23 
0 

0 

28 
33 

28 

16 
0 

0 

12 

5 
10 

20 
0 
23 
26 
22 
0 

0 

28 
0 
18 
25 
23 
0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

24 

"o 

11 
3 

0 

~0 
0 

18 
0 
0 

22 

18 

0 

0 

0 

0 

21 

20 

25 
27 
23 
22 

12 

20 
13 

8 

23 

0 
0 

18 

0 
0 

34 

0 
20 

5 
0 
1^ 

25 
0 
0 

19 
0 
0 

24 

2 

0 

0 

25 

10 

28 

13 

0 

0 

26 

13 

0 

0 

22 

27 

0 
30 
20 
20 

0 

18 

8 

19 

0 
0 

0 
0 

25 
0.7 

17 

0.5 

Season. 


No. 


Gr. 

fall 


92 
2.7 


21 


20 


15 


20 

18 
12 

5 
14 

18 

13 
13 

16 

17 
13 

18 


Table  XXVIII  shows  the  frequency  of  occurrence  of  a  fall  of  20°  or  more 
in  the  temperature  of  two  successive  days,  combined  with  the  attainment  of 
a  minimum  temperature  of  20°  or  less  in  December,  January  and  February, 
or  a  minimum  of  28°  or  less  in  March  and  November. 


MARYLAND    WEATHER    SERVICE  129 

As  a  special  chapter  is  to  be  devoted  to  an  analj'sis  of  cold  waves  in 
Part  II  of  this  Eeport,  reference  is  here  made  only  to  the  frequency 
of  their  occurrence.  In  the  period  comprising  34  winters  the  total  num- 
ber of  occurrences  fully  satisfying  the  technical  conditions  imposed  is 
92,  or  an  approximate  average  of  three  per  year.  Since  1871  there 
were  three  seasons  without  a  cold  wave,  namely,  1873-74,  1885-86,  and 
1889-90.  The  greatest  number  occurring  in  any  one  season  was  6,  in 
1871-72,  1884-85,  and  in  1903-4.  Of  the  total  of  92  in  34  years,  8  oc- 
curred in  JSTovember,  19  in  December,  23  in  Januar}^,  25  in  February, 
and  17  in  March.  The  greatest  fall  in  temperature  recorded  within 
the  prescribed  time  of  24  hours  was  38°,  which  occurred  in  December, 
1901.  Three  cold  waves  occurred  in  each  of  the  following  months: 
December,  1871-72,  January,  1898-99,  February,  1903-04,  and  March, 
1882-83. 

Killing  Frosts. 

A  factor  of  the  highest  importance,  especially  to  the  agricultural  and 
trucking  interests  of  a  community,  is  the  average  date  of  occurrence  of 
the  first  "  killing  "  or  "  black  "  frost  in  autumn,  and  the  last  in  spring, 
and  their  variations  in  time  of  occurrence  from  year  to  year.  Frosts  are 
usually  designated  as  "  light,"  "  heavy,"  or  "  killing."  The  term 
"  light "  is  applied  to  frosts  which  are  destructive  only  to  tender  plants ; 
"  heavy  "  to  copious  deposits  of  frost,  but  which  do  not  destroy  the  staple 
products ;  "  killing  "  to  such  as  are  blighting  to  the  staple  products  of  the 
locality  in  which  the  frost  occurs.  First  and  last  killing  frosts  are 
tabulated  below  for  each  year  from  1871  to  1904  for  the  vicinity  of 
Baltimore.  In  the  absence  of  a  killing  frost  before  a  minimum  tempera- 
ture of  32°  was  observed,  the  date  of  the  first  record  of  a  freezing  tem- 
perature was  entered  in  the  table.  The  interval  in  days  between  the 
last  frost  in  spring  and  the  first  in  autumn  is  likewise  given  in  order 
to  show  the  length  of  the  period  of  safe  plant  growth. 

The  average  date  of  occurrence  of  the  last  killing  frost  in  spring,  based 
on  observations  of  34  years,  is  April  4.  It  has  occurred  as  early  as 
February  26,  namely  in  1903,  and  as  late  as  May  3,  as  in  1882.  The  first 
killing  frost  in  autumn  has  occurred,  on  the  average  on  November  3. 


130 


THE    CLIMATE    OF    BALTIMORE 


The  earliest  appearance  is  that  of  October  6,  1892,  and  the  latest  that 
of  December  6,  1878.  In  the  ordinary  course  of  events,  accordingly,  the 
period  of  safe  plant  growth  in  the  neighborhood  of  Baltimore,  based 
upon  the  occurrence  of  killing  frosts,  is  from  April  4  to  November  3, 


TABLE  XXIX.-KILLING  FROSTS. 


18T1 

lS-2.... 

1873 

1374 

1875 


1876. 

1877. 
1878. 
1879. 
1380. 

1881. 
1883. 
138-3. 
1884. 
1885. 


1886.. 
1887.. 
1888.. 
1S89.. 
1890.. 


1891 . 
1892. 
1893. 
1894. 
1895. 


1896 

1897 *Mar. 

1S98 

1899 

1900 


1901. 
1902. 
1903. 
1904. 


Average  date  1871-1903. 

Earliest  date 

Latest  date 


Last  in  Spr 

mg-. 

First  in 

Autumn. 

Interval  in  days. 

Min. 

Min. 

*Feb. 

23 

30° 

*Nov. 

28 

31° 

*Mar. 

25 

33 

*    •> 

16 

30 

" 

31 

29 

Oct. 

29 

31 

OJ.7 

Apr. 

13 

29 

Nov. 

10 

31 

211 

" 

22 

3.' 

'* 

3 

33 

195 

.* 

» 

30 

Oct. 

15 

33 

196 

'• 

3 

33 

Nov. 

4 

37 

215 

Mar. 

2fi 

21 

Dec. 

6 

32 

255 

Apr. 

n 

32 

Oct. 

26 

30 

204 

■' 

12 

30 

Nov. 

8 

35 

210 

.. 

21 

39 

" 

27 

34 

220 

May 

3 

38 

" 

19 

30 

200 

Apr. 

25 

34 

" 

13 

32 

203 

Mar. 

30 

31 

" 

7 

30 

•;•» 

" 

IB 

31 

'^ 

1 

33 

2:30 

.. 

24 

29 

Oct. 

17 

36 

307 

Apr. 

6 

30 

*• 

31 

32 

308 

Mar. 

19 

30 

" 

22 

33 

217 

*      n 

30 

28 

Nov. 

6 

35 

*Apr. 

2 

31 

Oct. 

31 

36 

» 

9 

36 

" 

;:9 

33 

303 

" 

15 

34 

" 

6 

36 

1:4 

" 

16 

36 

" 

17 

36 

184 

" 

11 

32 

Nov. 

12 

''V 

31. 

** 

11 

34 

Oct. 

29 

34 

301 

" 

8 

33 

Nov. 

14 

32 

320 

*Mar. 

29 

34 

Oct. 

31 

39 

Apr. 

6 

26 

" 

28 

34 

205 

Mar. 

25 

;!0 

Nov. 

4 

38 

224 

** 

22 

26 

" 

16 

38 

239 

*   <i 

17 

30 

.1 

11 

31 

" 

31 

Oct. 

30 

34 

S37 

Feb. 

26 

29 

Nov. 

7 

28 

254 

Apr. 

17 

31 

■■ 

Apr. 

4  .... 

Nov. 

3    ... 

213  Average  period. 

Feb. 

26,  1903 

Oct. 

6,  1892 

355  Longest  period. 

May 

3, 

1883 

Dec. 

6. 

1878 

174  Shortest  period. 

*  No  frost  recorded  ;  first  day  in  Autumn  and  last  day  in  Spring  with  a  minimum  temi)er- 
ature  of  33°  or  below. 


or  approximately  seven  months.  While  this  is  the  most  probable  length 
of  the  period,  the  interval  may  be  considerably  extended  by  a  late  autumn 
frost  in  conjunction  with  an  early  spring  frost,  or  the  period  may 
be  shortened  by  a  late  sjjring  frost  followed  by  an  early 
autumn    frost.      The    extent    to    which    this    important    interval    has 


MARYLAND   WEATHER    SERVICE  131 

varied  in  the  past  34  years  is  shown  in  the  above  Table  XXIX.  The 
shortest  interval  namely,  5  months  and  24  days,  was  that  of  1892, 
extending  from  April  15  to  October  6;  the  longest  was  that  of  1878, 
extending  from  March  26  to  December  6,  or  8  months  and  15  days. 
Calculating  on  the  basis  of  a '34-year  record,  we  find  that  the  last  killing 
frost  in  spring  is  likely  to  occur  sometime  within  the  first  decade  of 
April  once  in  4  years ;  in  the  second  decade  once  in  5  years ;  in  the  third 
decade  once  in  11  years;  the  latest  occurrence,  as  stated  above,  was 
May  3,  in  1882.  In  the  autumn  the  first  killing  frost  has  occurred  but 
once  in  33  years  in  the  first  decade  of  October,  three  times  in  the  second 
decade,  and  ten  times  in  the  third  decade.  It  fell  within  the  first  decade 
of  Xovember  nine  times,  within  the  second  decade  seven  times,  and 
within  the  third  decade  twice.  The  Litest  in  33  years  occurred  on 
December  6,  1878. 

The  First  and  Last  Occurrence  of  a  Minimum  of  32°. 

Another  measure  of  the  period  during  which  staple  products  are  safe 
from  tlio  influence  of  low  temperatures  is  the  last  occurrence  of  a  re- 
corded temperature  of  32°  in  spring  and  the  first  in  autumn.  As 
these  records  are  determined  by  self-registering  instruments  they  are 
not  subject  to  the  uncertain  judgment  of  observers  as  to  the  character 
and  extent  of  damage  inflicted  by  the  frost.  Especially  is  this  judg- 
ment liable  to  error  in  the  case  of  observers  stationed  within  large  cities. 
With  a  fair  exposure  of  the  thermometer,  a  really  injurious  frost  is  not 
likely  to  occur  with  a  recorded  minimum  temperature  more  than  2°  or  3° 
above  the  freezing  point.  Tlie  average  of  the  minimum  temperatures 
recorded  at  the  time  of  occurrence  of  the  "  killing  frost"  in  tlic  preceding 
table,  is  31°.  It  sometimes  happens  that  a  temperature  several  degrees 
below  32°,  sufficient  to  form  heavy  ice  and  seriously  injure  vegetation, 
occurs  many  days  and  even  several  weeks  after  the  last  recorded  ''  killing 
frost."  Such  was  the  case  in  1903,  when  the  last  killing  frost  occurred 
on  the  2nth  of  February,  wliilc  a  temperature  of  27°  occurred  as  late 
as  April  5,  but  without  frost.  A  deposit  of  frost  requires  not  only  a 
freezing  temperature  but  a  liigli  iicirontagc  of  Inimidity  in  the  layers  of 


132 


THE    CLIMATE    OF    BALTIMORE 


atmosphere  resting  upon  the  ground.  Upon  the  basis  of  the  last  and 
first  "  freeze "  the  period  of  safe  vegetable  growth  is  lengthened  by 
about  13  days  as  compared  with  the  interval  between  the  last  and  first 
"  killing  frost."  Table  XXX  shows  an  average  interval  of  226  days 
between  the  last  occurrence  of  a  minimum  temperature  of  32°  in  spring 
and  the  first  occurrence  in  autumn.  AVith  killing  frosts  the  interval  is 
213  days.     In  the  exceptionally  warm  year  of  1871  the  interval  of  safe 


^^L 

__^_.^_ 

1   1   1  I  11 

___^___ 

r 

^^^^^ 

1    1    1    1 

-'--'— 

_. 

_ 

^r 

^ 

^^— 

p- 

- 

-J 

-f 

^ 

T 

tlTS 

1876 

1 

1       '    ' 

^^^^ 

-p 

1 

'    r 

.    . 

^^^^^^^ 

^ 

1  1  1   1   1 

j 

1    '  ^^^^ 

^^^^H      1 

i      '      ' 

' 

;  *^^ 

!    1 

^C  ^^M 

' 

■ 

.    ' 

_ 

J 

- 

^ 

H 

- 

- 

- 

- 

A 

- 

-i 

1  i 

1891 
1896 

2 

- 
- 

: 

-J 

- 

-1 

^^ 

.  - 

1 — 

1901 

~- 

\ 

\ 

] 

\ 

I 

: 
: 

■ 
- 

m 

— 

=j= 

-T- 

,    :    i    .    . 

I    .        1    1 

'    , 

^^^ 

'            ;           1           ' 

. , 

Fig.  33.  — Interval  Between  Last  and  First  Occurrence  of  a  Minimum  Temperature 
of  32°  (Heavy  Frost). 

Fig.  33  shows  the  time  of  occurrence  of  the  latest  freezing-  temperature  in  spring  and  the 
first  in  autumn;  also  the  intervening  interval  in  months  and  days,  for  each  year  Irom  1871 
to  19j3.  The  lowest  line,  marked  "mean"  shows  the  average  date  of  occurrence  and  the 
avei-age  length  of  the  intervening  period.    iSee  Table  XXX.) 


growth  wafi  lengthened  to  278  days.  In  1875  it  was  only  195  days. 
These  figures  indicate  that  under  the  least  favorable  conditions  during 
the  past  33  years  the  period  of  entire  safety  to  all  excepting  tender  plants 
near  Baltimore  was  six  months  and  a  half;  under  the  most  favorable 
conditions  a  trifle  over  nine  months;  while  the  average  period  is  seven 
months  and  a  half.  (See  also  Fig.  33.)  The  probability  of  given 
departures  from  the  average,  or  normal,  period  is  shown  by  the  follow- 
ing figures : 


MARYLAXD    WEATHER    SERVICE 


133 


FREQUENCY  OF  STATED  DEPAKTURE.S  FROM  THE  AVERAGE  LENGTH  OF  THE 
PERIOD  OF  SAFE  VEGETABLE  GROWTH. 

(Number  of  days.) 


Departure.               1-5 

ft-10 

11-15 

16-30  31-25 

1           1 
36-30  31-35  .36-10 

41-45 '4&-5O  51-55  Total 

Frequency 10 

In  percentage •••     30 

Cumulative  percentage    30 

5 
15 
45 

10 
30 
75 

3          1 
10          3 
85        88 

2          10 

6          3          0 

9t    i    97       .... 

i           1 

0          0          1    i    33 

0          0          3    '  100 

1  100    t  ■••• 

1           1 

TABLE  XXX.-LAST  AND   FIRST  OCCURRENCE  OF  A  MINIMUM  TEMPERATURE 

OF  32° 


Tear. 


Last  in  Spring. 


Feb.      Mar.      Apr, 


1871. 
1872. 
1S73. 
1874. 

1875. 

1376. 
1877. 

1878. 
1879. 
1880. 

1881. 
1882. 
1883. 

1884. 
1885. 

1886. 
1887. 
1888. 
1889. 
1890. 

1891 . 
1892. 
1893. 
1894. 
1895. 

1890. 
1897. 
189.S. 
I89f». 
1900. 

1901. 
1902 
l!)0;!. 
1904. 

Average  date,  1871-1880. 
••  ls^l-1890. 
"  1891-1900. 
"     1871-1903. 


31 


30 
29 


First  in  Fall. 


Oct.      Nov.      Dec 


13 
12 

U 
1- 
:.'4 
13 
14 

11 

29    I 
6 


278 
237 
243 
211 
195 

237 
232 
255 
204 
218 

231 
221 
226 
321 


228 
208 
S3: 
231 
233 

212 
210 
215 
215 

228 

220 
213 
231 
223 
217 

£39 
2.55 
215 


231 
236 
219 
226 


®.5 


+.53 
-1-11 

-1-17 
—15 
—31 

-hll 

+  6 
+29 

o*> 

—  8 

+  5 

—  3 
0 

—  5 

—  4 

+  2 
—18 
-i-11 
+  5 

+  7 

-14 
-16 
-11 
-11 

+  2 

—  6 
-14 
+  5 

—  3 

—  9 

-i-  1 
^2'.» 
-11 


The  average  departure  from  the  normal  period  of  226  days  is  about 
12  days.     In  the  year  18T1  the  last  freezing  temperature  occurred  53 


134 


THE    CLIMATE    OF    BALTIMORE 


days  before  the  average  date;  the  nearest  approach  to  this  excessive 
deviation  was  a  departure  of  31  days  in  the  opposite  direction  in  1875. 
The  figures  in  the  above  table  indicate  that  a  departure  exceeding  one 
month  is  extremely  improbable,  having  occurred  but  twice  in  3-1  years. 
In  28  out  of  a  total  of  34  years  the  limit  of  variability  was  under  30 
days;  in  25  it  was  under  15  days;  in  15  under  10  days;  and  in  10  under 
5  days. 


I 

, 

•   1 

^^^^^ 

- 



^    ; 

^^* 

!    1    1 

,  ^ 

1    i 

■    ' 

2*" 

'    1    1 

^^™ ' 

'                : 

! 

** 

^    ,    :    1 

,    .    ,    .   1 



_ 

H 

■B^MM 

^^-^ 

-t- 

- 

- 

- 

—- ^M 

"'~~^~^"r 

\~ 

- 

1    j 

- 

J_ 

'  !  1 

j 

[ 

1 

1 

■   ^^ 

1886 

._ 

_ 

_ 

-\ 

4S 

^■^^M 

I 

^ 

_ 



— 

-^ 

-- 

^ 

\  — 

- 

- 

-- 

1      ■ 

^^1  1 

^;  1 

1  ^1 

nss 

* 

!      '      '. 

i  1  i! 

■:  :  1 

MJJ 

■¥W 

Til 

^^ 

^Z 

1 

iffi 

L.           j_ 

1 

T 

-L 

1^1 

^^^■1 

^^ 

■^■21 

"□"•" 

,  '    .  , 

■  H^^E  ■ 

,  , 

'II 

1  ,  1 

i  1  1 

1 

M 

1        1    j    1 

I.I 

1     .     .     i.    ^. 

±1± 

MEAN 

1  1  !  !  1 

■r*T 

L 

. 

il 

McH.  Apb-  May  June  J"' v  Aug.  Sept.  Oct.  Nov.  Dec. 

Fig.  34. —  Interval  Between  Last  and  First  Occurrence  of  ^Minimum  Temperature 
of  40°  (Light  Frosti. 

Fig-.  34  shows  the  time  of  last  occurrence  of  a  minimum  temperature  of  40°  in  spring,  and 
that  of  the  first  occuri-ence  in  fall,  together  with  the  length  of  the  intervening  period  in 
months  and  daj's.  The  lowest  line  marked  "  mean"  shows  the  date  of  average  occurrence 
and  tbe  average  length  of  the  intervening  period.    (See  Table  XXXI. i 

The  probability  of  the  occurrence  of  an  injurious  freeze  some  time 
within  the  month  of  April  may  be  expressed  by  65  on  the  basis  of  a 
possible  100,  a  temperature  of  32°  or  less  having  occurred  at  some  time 
within  the  month  22  times  in  34  years.  The  probability  of  occurrence 
from  the  1st  to  the  10th  of  the  month  is  41 ;  from  the  11th  to  the  20th, 
21;  from  the  21st  to  the  31st,  3.  It  has  occurred  15  times  in  34  3'ears 
on  some  day  between  the  1st  and  the  l(^th  of  April;  7  times  from  the 
11th  to  the  20th;  and  1  time  after  the  20th.  These  figures  represent 
the  chances  of  an  injurious  freeze  in  the  first,  second  and  third  decades, 
respectivel}',  of  the  month  of  April. 


MAKYLAXD    WEATHER    SERVICE 


135 


Light  Frosts. 

A  light  frost,  injurious  onh'  to  tender  plants,  may,  and  frequently 
does,  occur  with  a  recorded  minimum  temperature  of  40°.  Hence  the 
period  of  safe  growth  for  the  frailer  varieties  of  vegetation  is  reduced 

TABLE  XXXI.— LAST  AND  FIRST  OCCURRENCE  OF  A  MINIMUM  TEMPERATURE 

OF  40°. 


Last  in  Spring^. 


Year. 


Mar.      Apr.      May 


ISTl 
1ST3 
1873 
1874 
1875 

1876 

1877. 
1878 
1879 
1880 

138L 
1S82 
1883 
1884 

1885. 

1886 
1887 
1888. 
1889 
1890 

1391 
1892 
1893 
1894 

1895 

1896 
1897 
1898 
1899 
1900 

1901 
1903 
1903 
1904 

Average  date,  1871-1880 
"  I8fl-1890 
"  1891-1900 
"      1871-1903 


13 


13 


First  in  FalL 

_  o6 

Sept. 

Oct. 

Nov. 

Sa    i 

5"^   t 

31 

30(1 

13 

179 

is 

145 

15 

168 

13 

140 

8 

157 

4 

203 

39 

244 

36 

161 

35 

177 

15 

308 
184 

itJ 

lf,9 

33 

194 

23 

l-ifi 
■.'10 

3 

160 

9 

185 

•2S 

191 

18 
3 

165 
160 

17 

173 

15 

185 

9 

149 

9 

183 

18 

180 

17 

171 

1 

167 

17 

160 

18 

189 

29 

196 

19 

170 

17 

178 

23 

186 

12 

169 

18 

179 

a": 


+37 
0 
-»4 
—11 
-39 


+34 
+6.-. 
—18 


+39 
+  5 
—10 
+15 


+31 

—  3 
—19 
+  6 
+  13 

-14 
-19 

—  ti 
+  6 
-30 

+  3 

+  1 

—  s 
—13 
-13 

+10 
+17 


—  1 

+  7 
-10 


still  more.  A  minimum  of  40°  has  been  recorded  at  Baltimore  as  late 
as  May  26,  namely  in  1875;  but  this  is  an  exceptional  case.  The  last 
spring  mini iiui 111  of  40°  has  occurred  as  early  as  March  29.  The 
dates  of  the  last  in  spring  and  first  in  autumn,  witli  the  length  of  the 


136 


THE    CLIMATE   OF    BALTIMORE 


intervening  interval  in  days,  is  shown  in  Table  XXXI.     This  interval 
is  also  shown  graphically  in  Fig.  34. 

The  Period  of  Effective  Temperatures  for  Plant  Growth. 

It  is  generally  conceded  that  every  plant  requires  a  certain  tempera- 
ture in  order  to  develop  successively  the  leaf,  bud  and  fruit ;  that  there  is 
a  minimum  temperature  below  which  physiological  activity  in  the  plant 
ceases,  and  hence  that  only  temperatures  above  this  limit  are  effective 
in  carrying  the  plant  forward  from  the  sprouting  of  the  seed  to  the 
maturity  of  the  fruit.  This  "critical"  point  in  the  history  of  plant 
growth  is  assumed  to  be  a  mean  daily  temperature  of  43°  F.     In  order 


1875 

1880 

1885 

1890 

189b 

1900 

DAYS 

- 

L 

- 

200 

- 

- 

- 

- 

- 

_ 

DAYS 
300 


Fig.  35.  — Annual  Number  of  Days  with  Mean  Temperature  above  ■42°. 

Fig.  35  shows  the  total  annual  number  of  dajs  having  a  mean  temperature  of  43°  or 
above,  the  degi-ee  of  heat  which  marl£S  the  beginning-  of  physiological  activit)-  in  plants. 

to  determine  the  number  of  days  in  the  year  during  which  this  "  effec- 
tive "  temperature  prevails  in  the  vicinity  of  Baltimore,  the  days  with  a 
mean  daily  temperature  of  43°  or  above  from  1871  to  1903  have  been 
tabulated.  The  normal  number  of  such  days  for  each  month  and  for 
the  year  is  shown  in  the  following  table,  while  the  variation  in  the  total 
annual  number  is  represented  in  Fig.  35. 

PERIOD  OF  effective  TEMPERATURES  FOR  PLANT  GROWTH. 
(Average  number  of  days.) 


Means. 

Jan. 

Feb.  Mar.  Apr. 

May 

.Tune  July 

Aug. 

Sept.   Oct.    Nov.  Dec.  Year 

1871  1902 

5.3 

0.5      14.1     27.0 

31. n 

30.0  ;  31.0 

31.0 

30.0     30.0     19.4     8.2       264 

MARYLAXD    WEATHER    SERVICE 


137 


The  first  appearance  of  a  daily  mean  temperature  of  43°  in  spring 
normally  falls  upon  the  25th  day  of  March,  and  the  last  upon  the  27th 
day  of  November,  an  interval  of  245  days,  or  about  eight  months.  How- 
ever, such  days  occur  throughout  the  year  and  are  probably  effective  in 
directly  or  indirectly  promoting  physiological  activity  in  the  plant. 
Hence  to  the  period  above  mentioned  must  be  added  the  days  of  the 
winter  and  early  spring  months  before  the  permanent  appearance  of 
the  daily  mean  of  43°.  This  will  materially  increase  the  annual  period 
of  "  effective  "  temperatures,  as  a  considerable  proportion  of  the  winter 
days  fall  within  the  prescribed  limit.  Calculating  on  this  basis  the 
average  period  comprises  264  days.     The  longest  period,  namely,  that 


1675 

1880 

1885 

1890 

1895 

1900 

DAYS 

10 

Fig.   36. — Annual  Number  of  Days  with  Maximum  Temperature  of  90°  aud  over. 
(See  Tables  XXXII  and  XXXIII.) 

of  the  year  1878,  contained  293  days,  and  the  shortest,  244  days  in  1886. 
The  ten  year  average  values  of  the  three  decades  from  1871-1900  varied 
only  from  261  days  to  268  days,  showing  a  remarkably  constant  average 
length  for  this  period. 


The  Frequency  of  Warm  Days  ix  Summer. 

It  is  a  well  recognized  fact  of  observation  that  the  temperatures  above 
the  normal  heat  of  a  locality  fluctuate  to  a  less  degree  than  the  tempera- 
tures below  the  normal.  In  other  words  the  extent  of  variability  in  the 
temperature  for  a  given  locality  is  generally  determined  by  the  cold 
days  and  not  by  the  warm.     The  extreme  maximum   temperature  in 

the  United  States  has  a  range  of  about  40°,  or  from  80°  to  120° ;  the 
10 


138 


THE    CLIMATE    OF    BALTIMORE 


extreme  miniiiuim  varies  from  (15°  below  zero  to  about  40°  above,  a 
range  of  105°.  While  variability  in  the  temperature  of  warm  da3-s  is  of 
less  importance  in  agricultural  and  mercantile  life  than  that  of  cold  days, 
it  is  a  faetoi-  of  much  concern  in  personal  comf(jrt,  especially  to  the 
dwellers  in  large  cities. 

TABLE  XXXII.-LIST  OF  WARM  DA YS.-Aprii.. 
(Temperature  of  90°  or  above.) 


Year. 

' 

2 

3 

4 
o 

5 
o 

6 
o 

7 
o 

8 
o 

9 
o 

10 

0 

11 
o 

12 
o 

13 

o 

14 
o 

15 

0 

1617 

0    0 

181920 

o  o  o 
9493;; 

21 

0 

22 

0 

2334253637 
00000 

3829 

0  0 
..90 

30 

0 

9i 

1 

0 

1888 

1898 

o 

o 

o 

1 

1903 

1 
4 

May. 


1 

0 

0 

0 

3 

0 

4 
0 

5 
0 

6 

0 

7 
0 

8 
0 

9 

0 

10 

0 

11 
0 

13 
0 

13 
0 

14 
0 

15 

16 

17 

18 

19 

20 

•21 

22 
0 

33 

0 

90 

34 

0 

35 
0 
90 

•■ 

26 

0 

■■ 
92 

37 
0 
93 

38 
0 

90 

29 
0 

30 
0 

95 

31 

0 
94 

95 

0 

1877 

0 

0 

0 

0 
92 

90 
92 

0 
93 

0 

9i 

•• 
92 

0 

0 

1879 

18S0 

•• 

91 

9i 

94 

94 

!? 

•• 

90 
93 

1 
7 

18S1 

1 

3 

1889 

90 
93 

93 
96 

0 

1895 

1«!)6 

__ 

:: 

3 
5 

189S 

1 

1899 

1900 

.. 

9i 

9i 

94 

1 
3 

19U-,' 

1 

1903 . 

92 

1 

39 

Table  XXXII  contains  a  complete  list  of  all  days  from  1871  to  1903,  during 
which  the  temperature  rose  to  90°  or  above;  together  with  the  maximum 
temperature  recorded  on  such  days  and  the  monthly  frequency  of  occurrence. 
Fig.  36  shows  the  annual  frequency  of  days  upon  which  the  maximum  tem- 
perature equalled  or  exceeded  90°. 


The  frequency  of  occurrence  of  days  with  an  excessively  high  tempera- 
ture hence  plays  an  important  part  in  the  composition  of  local  climates. 
We  can  provide  against  extreme  cold  in  Avinter;  from  the  hot  and  ener- 
vating summer  weather  there  is  no  escape  for  the  great  numbers  who 
are  compelled  by  circumstances  to  remain  in  the  large  cities  beyond  the 
reach  of  mountains  or  seashore. 

A  temperature  below  90°  is  not  especially  uncomfortable  or  unwhole- 
some unless  accompanied  Ijy  a  high  degree  of  humidity  and  a  stagnant 


MAEYLAND   WEATHER   SERVICE 


139 


atmosphere.  Defining  a  hot  day  as  one  with  a  temperature  of  90°  or 
above,  the  Baltimore  statistics  show  a  frequency  and  distribution  indi- 
cated in  Table  XXXII. 

TABLE  XXXII.-LIST  OF  WARM  DATS.-June. 
(Temperature  of  90°  or  above.) 


Year. 

1 

0 

0 

3 

0 

4 

0 

5 
o 

6 
o 

7 
o 

8 
o 

9 

o 
90 

10 

o 

11 

0 

13 

0 

90 

13 

o 

14 

o 

15 

0 

16 
o 

17 

o 

18 
o 

19 

0 

93 

20 
o 

21 

0 
9i 

22 
0 

23 
0 

95 
96 

24 
0 

93 
96 

90 

95 
9i 

93 

98 

(V) 
94 

25 
0 

95 

90 

92 

93 
97 

92 
93 

92 

9S 

36 
.° 

9i 

97 

94 
95 

90 
92 

93 
91 

95 
92 

27 
0 

94 

95 

92 
92 
91 

90 

9;i 

92 

38 

0 
91 
92 

93 
93 

93 

92 
94 

92 

94 
9i 

93 

29 
0 

97 
90 

93 

93 
91 

0 
94 

.. 

95 
90 

99 

1871 

1 

3 

^ 

4 

90 

96 

98 

90 

9 

5 

7 

6 

5 

90 

90 

95 

92 

90 

93 

91 
9i 

9i 
90 

yi 
90 

92 

.. 
92 
91 

90 
90 

93 

93 

93 

93 

97 
91 

4 

H . 

9 

94 

1 

5 

1880 

9 

1 

3 

o 

90 

90 

n 

;j 

90 

1 

4 

4 

5 

91 

95 

90 

94 

9S 

94 

4 

6 

0 

n 

92 

94 

90 

s 

9 

0 

1890 

91 

90 

93 

94 

93 

94 

90 
91 

9i 

90 

9i 
9:i 

4 

1 

90 

n 

R 

3 

4 

90 

92 
90 

90 

5 

5 

9t 

95 

97 

5 

6 

H 

7 

8 

90 

93 

9i 

92 
91 

94 

90 

93 

•• 

93 

2 

7 

9 

93 

98 

96 

95 
91 

8 

1900 

n 

1 

7 

93 

4 

3 

0 
147 

Such  days  do  not  generally  make  their  appearance  until  the  month  of 
June  and  disappear  in  the  first  week  of  September.  The  average  num- 
ber for  the  entire  season  is  21,  and  the  monthly  distribution  is  as 
follows:  June  4,  July  10,  August  5,  and  September  2.  They  occasion- 
ally occur  in  May  (about  one  in  two  years  on  the  average),  and  have 
occurred  on  two  occasions  in  33  years  as  early  as  April  and  once  as  late  as 
October.     There  is  considerable  difference  in  their  total  frequency  during 


140 


THE    CLIMATE    OF    BALTIMOEE 


a  period  of  10  years:  From  1871  to  1880  there  were  204;  from  1881  to 
1890,  161 ;  from  1891  to  1900,  243.  The  annual  frequency  has  varied 
from  8  in  1871  to  43  in  the  memorable  summer  of  1900,  as  shown  in 
Fig,  36.     It  is  remarkable  that  the  summer  containing  the  highest  total 


TABLE  XXXII.— LIST  OF  WARM  DAYS.— July. 

(Temperature  of  90°  or  above.) 


Year. 

1 

2 

3 

4 

5 

0 

93 
91 

92 

92 
92 

6 
o 

94 
90 
92 

7 

o 
90 

90 

8 
o 

92 

97 
93 

9 

o 
91 

92 

99 
91 
91 

93 
90 

10 

o 
91 
93 

96 

97 

94 
93 
97 

93 

11 

o 

90 
96 

oi 

95 

94 

'M 

13 

° 

96 

90 

13 

o 
91 

90 
95 

'M 

Itfi 

9i 

96 

9i 
94 
97 

93 

93 

14 

o 

92 
94 
92 

91 
91 

90 
it;.' 

95 
90 

90 
91 

9i 

15 

0 

90 

9i 
94 

91 

92 

94 
Itl 

9i 

90 

9i 
96 

92 

16 

o 
92 
91 
90 

9i 

90 
91 

99 
94 

91 
93 

loi 

92 

92 

92 

100 

90 

17 

0 

9i 

93 

90 

9i 
91 
90 

95 
99 
97 

90 
90 

100 
9fi 

18 

o 

95 

96 

96 
93 
98 

96 
102 

92 
93 

98 

90 
99 

19 
o 

93 
96 

93 

94 
93 

20 

0 

9i 

97 
93 

98 
90 

97 

93 

90 

9i 

90 
95 

21 

0 

95 
99 

95 

92 
94 
96 

92 

33 
o 

90 

96 
93 

■■ 

9i 

96 
93 

33 

0 

90 

93 

95 
93 
92 

90 

•  • 
94 

94 

o 
92 

90 
91 

95 
92 

90 

93 
90 

25 
o 
94 

92 

90 
90 

96 

97 
92 
91 

9i 

94 

9i 

26 

0 

94 
93 

94 

93 
91 

90 
93 
94 

99 
98 
93 

■• 
94 

37 

0 

93 
90 

9i 

97 
92 

96 

92 

38 
o 

92 
93 

95 
97 

92 

90 

29 

0 

90 
93 

97 
90 
95 

94 
94 

96 
92 

30 
o 
9i 

92 

91 
92 

92 

9i 

92 
94 

95 
95 

31 

0 

90 

95 
90 

•  • 
90 

o 

1871   

0 

96 

o 

97 
93 

0 

96 
96 

0 

93 
93 
92 

s 

1" 

3 

15 

4 

7 

1 

6 

94 

93 

95 

92 

98 

IS 

93 

^ 

H 

16 

9 

95 

10 

1880 

10 

1 

91 

93 

96 

96 

91 

11 

8 

3 

91 

92 

94 

94 

93 

7 

4 

s 

5 

91 

93 
94 

93 
90 

93 
98 

94 

93 
90 

93 

90 
90 

92 
90 

If) 

g 

i 

7 

8 

90 

9i 

10 
5 

9 

») 

1890 

91 

8 

1 

0 
10 

3 

91 

90 

93 
95 

90 

96 

90 
94 
95 

95 

92 

9i 
93 

q 

4 

91 

11 

5 

5 

6 

90 

104 
90 
92 

97 
97 
95 

l66 
90 
97 

96 

94 

94 
95 

90 

96 

96 
95 

96 
90 

9i 

93 

92 
90 

10 

91 
97 

<) 

8 

100 

10 

9 

8 

1900 

15 

1 

103 

103 

19 

3 

10 

3 

93 

95 

1'^ 

307 

number  of  excessively  hot  days  in  a  period  of  33  years  did  not  have  an 
average  temperature  sufficiently  high  to  class  the  season  as  excessively 
warm.  The  "  hot-spell "  occurred  late  in  July  and  in  August  and  was 
followed  by  an  exceptionally  warm  autumn.  This  period  will  receive 
further  attention  in  Part  II,  in  the  discussion  of  summer  weather. 
Inspection  of  Fig.  36  reveals  a  strikingly  uniform  increase  in  the  num- 


MARYLAND   WEATHER    SERVICE 


141 


ber  of  days  with  a  maximum  temperature  of  90°  and  above  since  the  year 
1889.  Starting  with  a  frequency  of  9  in  the  latter  year,  the  number 
rose  steadily  to  43  in  1900  with  but  one  marked  interruption  and  two 
or  three  minor  ones.     Since  1900  there  has  been  a  steady  fall,  repre- 

TABLE  XXXII.-LIST  OF  WARM  DAYS.-AuGUST. 
(Temperature  of  90°  or  above.) 


Year. 

1 

o 

2 

o 

3 

o 

4 
o 

5 

o 

e 

o 

7 

0 

8 

0 

9 
o 

10 
o 

11 

0 

12 

0 

13 

0 

14 

o 

15 

o 
90 

16 

o 
9J 

17 

0 

18 

0 

90 
9i 

19 

0 

93 

92 
92 

92 

20 

o 
93 
97 

92 
94 

97 

21 

o 

gi 

96 

" 
•• 

91 
90 

97 
90 

22 

o 
96 

90 
90 

23 
o 
90 

90 
93 

24 

0 

94 

94 
94 

93 

25 

0 

90 
92 

90 
97 

26 

0 

92 

90 
96 

27 

0 

92 
90 
92 

28 

0 

90 

90 
9i 

29 
o 

94 
9i 

92 

30 
o 

9i 

31 

0 

91 

96 
93 

"5 
o 

1871 

o 

92 

92 

94 

95 

96 

90 

1*? 

3 

4     

94 

91 

90 

4 

3 

5 

6 

90 

92 

92 

9i 

93 

98 

90 

0 

g 

8 

91 

90 

90 
94 

90 
94 

90 
93 

90 

(f 

9 

qoqo 

r, 

1880 

1 

92 

2 

8 

1 

3 

p 

4 

3 

5      

90 
92 

90 

oi 

90 
90 

96 

90 
9i 

90 
90 
9i 

92 
92 

9i 

3 

6 

90 
91 

94 

92 

9i 

91 
90 

9i 

97 

4 

90 
90 

93 

94 

90 

92 
95 

94 
95 

s 

8 

90 

94 

93 

10 

9 

1890 

95 

1 

9 

1 

5 

3 

3 

9 

4 

93 
91 

94 

90 
94 

97 

96 
90 

95 
96 

o 

6 

10 

6 

91 

96 

94 

98 

10 

1 

8 

93 
91 

93 

97 

93 
100 

92 
99 

l66 

100 
93 

i66 

90 

99 

gi 

92 

9 

9 

1900 

90 

90 

8 
17 

1 

8 

•J 

3 

90 

91 

90 

•• 

4 
2 

m 

sented  by  the  figures  43,  30,  20,  16.  The  numbers  for  the  rising  branch 
of  the  curve  of  frequency  are:  9,  14,  11,  19,  16,  23,  28,  29,  12,  35, 
27,  43. 

The  real  discomfort  of  a  hot  summer  depends  not  so  much  on  the 
actual  number  of  hot  days  as  the  length  of  the  periods  of  uninterrupted 
hot  weather.  A  month,  for  example,  with  10  scattered  days  having  a 
temperature  exceeding  90°  would  be  far  more  comfortable  than  one  with 
an  equal  number  of  consecutive  days  with  the  same  degree  of  heat.     In 


14S 


THE    CLIMATE   OF   BALTIMORE 


fact,  liot  spells  of  tlie  latter  description  are  of  comparatively  rare  occur- 
rence in  Baltimore,  not  having  occurred  more  than  five  times  in  33  years, 

TABLE  XXXIL-LIST  OF  WARM  DAYS.-September. 

(Temperature  of  90°  or  above.) 


Year, 

1 

2 

3 

4 

5 

f) 

7 

8 
o 

9 
o 

10 

0 

11 

o 

12 

0 

13 

o 

14 

o 

15 

0 

16 

0 

17 

0 

18 
o 

90 
9i 

19 
o 

■■ 
94 

20 
o 

90 

21 

o 

96 

o 
96 

23 
o 

95 

o 

25 
o 

90 

26 
o 

90 
93 

27 
o 

90 

28 
o 

91 

29 

0 

30 
o 

-2 

1871 

o 

o 

0 

o 

o 

o 

o 

0 

2 

94 
90 

93 

92 
92 

98 

90 
'  1 

o 

3 

P3 

90 

o 

4 

o 

5  

92 

90 
9i 

9i 

90 
93 

90 

101 
90 

>) 

6 

7 

8 

9 

1880 

0 
0 
0 
0 
1^ 

1 

R 

2 

3 

i 

0 
0 

5 

0 

6 

90 

•• 

t 

7 

8 

9 

1890 

1 

0 
0 
0 
0 

1 

2 

3 

4 

94 

94 

90 
97 
95 

93 

93 

92 

[ ' 

■■ 

0 
0 

o 

5 

7 

6 

91 

93 
94 
94 

93 
9i 

90 

9i 

91 

92 

3 

7 

4 

8 

95 

96 

97 

91 

90 

8 

9  

o 

1900 

90 

4 

1 

1 

9 

92 

" 

1 

3 

n 

m 

October. 


1897. 


1011 


192021222324252627 


293031 


while  the  former  condition  has  occurred  about  25  times.  A  list  of  the 
longest  periods  of  consecutive  days  with  a  maximum  temperature  of  90° 
or  above  for  each  year  from  1871  to  1903  is  published  in  Table  XXXIII. 
The  table  likewise  shows  the  dates  of  beginning  and  ending  of  the  periods 


MARYLAND    WEATHER    SERVICE 


143 


and  the  maximum  temperatures  attained.  Their  annual  average  length 
is  a  little  less  than  six  days,  with  limits  of  variations  hetween  2,  as  in 
1871,  1886  and  1889,  and  14  in  1900.  The  season  with  the  longest  hot 
spell  on  record  likewise  contained  some  of  tlie  highest  temperatures  ever 

TABLE  XXXIII.-LONGEST  PERIOD  OF  CONSECUTIVE  DAYS  WITH  A  MAXIMUM 
TEMPERATURE  OP  90°  OR  ABOVE. 


1871 

1872 

187.3 

1874 

1875 

1876 

1877 

1878 

1879 

1880 

1881 

1883 

188;^ 

1884 

1885 

1886  .  .. 

1887 

1888 

1889 

1890 

1891 

1893 

1893 

1894 

1895 

1896 

1897 

1898 

1899 

1900 

1901 

19f)2 

1903 

Average 


Began. 


July  9 
June  30 
July  14 
June  7 
June  23 

July  8 
July  25 
July  4 
Aug.  2 
July    9 

July  3 
July  34 
July  2 
June  19 
July  16 

July  7 
July  11 
Aug.  3 
July  8 
July  30 

Aug.  9 
July  34 
June  19 
July  2.i 
May  30 

Aug.  4 
Sept.  9 
Aug.  30 
.lune  5 
Aug.    6 

June  26 
Aug.  2 
July    8 


Enrled. 


July 


Aug.    1 

12 
31 
23 

29 
•lune   3 

13 
11 

Sept.  1 
8 
19 

July    7 

4 

11 


Length. 


Days. 


Max.  temp. 


8 
4 
11 

*  2 
4 

*  2 

*  3 


10 

3 

9 

*  4 

14 

12 
3 
4 

5.8 


91° 


93 
94 
92 
99 

96 
93 
94 
93 
99 

92 
96 
94 
93 
95 

94 
99 
98 
97 
97 


97 
97 

98 
100 

103 
91 
96 


♦Two  periods  of  equal  length  ;  the  period  with  the  highest  maximum  temperature  selected. 


recorded  in  Baltimore.  On  six  successive  days  the  maximum  tempera- 
ture ranged  between  99°  and  100°;  the  month  contained  17  hot  days, 
and  was  preceded  by  a  iiiiuitli  with  I't.  This  was  doubtless  the  most  try- 
ing period  in  tlic  history  of  Pialtimore  summers.  A  more  extended  ac- 
count of  tliis  roniarkal)l('  hot  s])ell  will  l)e  given  in  V^vi  IF  of  this  Report. 


144 


THE    CLIMATE   OF   BALTIMORE 


-EEE:::::!;i|^ 

1 

5 

i 

I 

il: 

■;  1  '  1  - 

^ 

Fig.  37.  — Time  of  Occurrence  of  the  Lowest  aud  Highest  Temperature  of  the 
Year. 

Fig.  37  shows  the  time  of  occurronce  of  the  lowest  temperature  of  each  winter  season, 
trom  18a  to  1904,  and  of  the  highest  temperature  of  each  succeeding' summer  season,  from 
18/1  to  1903;  also  the  length  of  the  intervening  period  in  months  and  days.  The  lowest  line 
marked  '  mean  "  shows  the  average  time  of  occurrence  and  the  average  length  of  the  inter- 
vening period.  The  line  for  1904  shows  only  the  time  of  occurrence  of  the  lowest  tempera- 
ture.   (See  Table  XXXIV.) 


^ 

/ 

// 

,- 

\ 

// 

// 

■^^ 

\ 

/ 

1 1 

// 

1 1 
11 

1 

\ 

1 

I 
// 
1 

\ 

/ 

II 
1 
1 
'/ 

\ 

/ 

// 

/ 

b  /'  c 

\ 

/: 

s 

V 

■"^ 

"*> 

Fig.   38.— (a)  Air  Temperature  at  2  p.   m.      (Harbor). 

(6)  Temperature  of  Surface  Water,  2  p.   m.     (Harbor), 
(c)  <<  "    Water  at  depth  of  10  ft.     (Harbor). 

Fig.  38  shows  the  mean  monthly  temperature  of  the  water  in  the  harbor,  and  of  the  air  at 
2  p.  m.,  at  the  foot  of  youth  street,  (a)  Air  temperature ;  (70  temperature  of  the  water  at 
the  surface ;  (c)  temperature  of  the  water  at  the  bottom  (10  ft).    See  Table  XXXV. 


MARYLAND  WEATHER  SERVICE 


145 


Time  of  Occurrence  of  Annual  Minimum  and  Maximum 
Temperatures. 

The  dates  of  occurrence  of  the  winter  minimum  and  the  summer  maxi- 
mum temperatures,  and  the  length  of  the  intervening  period,  are  ex- 


table  xxxiv.-time  of  occurrence  of  annual  minimum  and  maximum 

temperatures. 


Min. 

Date  of  minimum. 

Date  of  maximum. 

Max. 

Summer. 

Dec. 

Jan. 

Feb. 

Mar. 

June 

July 

Aug. 

Sept. 

1870-1  

18T1-L' 

\^-,o  3 

10° 
5 
—4 

13 
-2 

13 

I 

0 
13 

—6 

7 

11 

8 

3 

—1 

7 
9 
3 
13 

16 
13 

1 
8 

1 

5 

8 

10 

8 

13 

11 

5 

si 

18 
10 

37 

23 

5 
8.04 

30 

16* 

10 

's 

3 

1 
24 
23 

6 

■3 

23 

i7 
16 

26 

6 

15 
4.08 

6 

ii 

5. 
24 

25 
6 

17 

0 
10 

1 

io 

11 
4.01 

"7 
3 

6 

3 
13.07 

'9 
27 

26 
25 

26 
24 
3 

"e 

8 
B7.«2 

16 
0 

18* 
9 

is 

16 
13 

23 

24 
21 

7 
18 

'9 

8 

26 
'3 

It 

18 

19 
97.°9 

ie 
11 

7 

25 

4 
96.03 

7 

ii 

3 
99.00 

930 

97 

96 

98 

97 

99 
95 
98 
99 
99 

101 
97 
96 
95 
99 

93 
102 
96 
93 

98 

94 
99 
98 
98 
97 

98 
97 

104 
98 

100 

103 
99 
97 

1871 

1872 
1873 

l}Jt73  4 

1874 

l»T4-5 

1875-6  

1875 
1876 

1S76-7  

1877 

1877-8  

1878 

1878-9  

1879 

1879-80 . .    

1880-1 

lt<8l  •' 

1880 

1881 
1883 

1}<H'>  3 

1883 

1883-4  

1884-5  

1884 
1885 

1885-6  

1886 

18iS6-7 

1887 

1887-8  

1888-9  

1889-90 

1885 
1889 
1890 

Iy90-1  

1891 

1891  3  

1893 

189"^  3 

1893 

1893-4 

1894-5  

1894 
1895 

1895-6  

1896 

1896-7  

1897 

1897-8  

1898-9    

1899-1900 

1900-1  

1901-"  

1898 
1899 
1900 

1901 
1903 

iyo-_'-3 

1903 

1903-4  

1904 

No.  of  occurrences 

•"•• 

*  On  other  days  also. 

ceedingly  variable  quantities.  While  the  lowest  winter  temperature 
usually  occurs  in  January,  it  has  on  several  occasions  appeared  in 
December,  frequently  in  February,  and  occasionally  in  March,  in  the 
past  34  years.  The  earliest  occurrence  was  on  the  10th  of  December 
in  187G,  the  latest  on  the  7th  of  March  in  1890. 


146 


THE    CLIMATE   OF    BALTIMORE 


TABLE   XXXV.-MEAN   TEMPERATURE  OF  AIR  AND    OF   SURFACE  WATER    IN 

THE  HARBOR  AT  3  P.  M. 


Date. 


n 


Air 
Water 


10 
II 

W 

15;- 

16^ 


18- 


20 


28 


Jan.  Feb.  Mar.  Apr.  May  June  July  Aug.  Sept.  Oct.  Nov.  Dec 


Monthly  Average  Air  ...  «« 
Water.  Si 


35 


36.3 

SA.3 
35.3 

Si.O 
40.5 

i^ii.e 

35.2 

Si.6 
37.0 
54.6 
89.3 
Si.l 
45.0 
Si.O 
43.7 
Si.O 
43.1 
Si.i 
41.0 
Si.S 
35.7 
SJ,.5 
43.7 
Si.S 
46.7 
35./, 
45.6 
36.1 
45.6 
56.6 
43.3 
36.3 
44.9 
56.  i 
41.9 
36.5 
41.0 
56. :? 
35.5 
5,5.7 
38.3 
56.0 
38.0 
36.9 
38.3 
,36.6 
36.8 
56.6 
44.3 
56.7 
40.0 
.1)6.6 
43.3 
36.3 
43.3 
57.5 


4    40. 

.8     35. 


42.1 

37.1 
46.3 
57.7 
43.6 
.37.5 
43.7 
56.  S 
42.4 
57.2 
43.3 
37.9 
41.7 
5S.0 
35.7 
5«.3 
39.3 
58.  i 
46.6 
5«.5 
43.7 
55.7 
48.1 
3S.S 
45.4 
39.5 
47.0 
59.9 
.50.9 
40.5 
46.8 
40.  i 
45.3 
40.2 
46.9 
40.6 
49.9 
40.9 
41.9 
40.9 
45.3 
40.5 
41.2 
40.6 
39.3 
/.1.2 
47.3 
4i.7 
49.2 
42.5 
53.6 
42.6 
53.5 
45.0 
53.8 
45.9 
46.7 
45. i 
48.6 
42.S 
53.3 
45.1 


54.1 

iS.O 
58.4 
45.7 
62.9 
46.0 
51.0 
46.0 
55.1 
46.6 
55.3 
47.0 
66.8 
47.6 
55.5 
47.9 
61.8 
47.6 
53.5 
iS.l 
48.3 
iS.7 
.54.9 
45.5 
58.1 
49.5 
57.3 

50.4 

54.3 

50.6 
57.5 
5i.4 
59.3 
5i."4 
63.9 
52.0 
6.).  2 
55.2 
64.6 
54.  i 
63.6 
5/t.5 
63.8 
55.3 
61.5 
55.5 
65.0 
55.9 
59.9 
56.5 
63.1 
56.  i 
65.8 
57.0 
67.5 
57.5 
61.4 
57.4 
63.5 
57.6 


45.8    68.7 
40. -Z     51. i 


63.8 

57.6 
68.3 
58.3 
65.8 
59.0 
69.7 
.59.5 
69.0 
59.2 
65.9 
59.2 
61.6 
59.4 
63.9 
59.6 
73.3 
59.6 
69.0 
60.8 
64.6 
60.6 
63.0 
6i.O 
69.0 
60.5 
60.9 
60.5 
68.1 
60.8 
67.1 
6?.0 
66.2 
6i.O 
69.0 
61.6 
69.1 
62.5 
71.5 
62.8 
73.3 
64.^ 
73.1 
64.4 
73.3 
66.1 
71.9 
65.6 
70.0 
65.0 
71.6 
65.2 
72.9 
65.5 
73.8 
65.7 
69.7 
66.2 
70.4 
66.8 
73.2 
67.0 


62.7 


78.1 
67.4 
74.4 
67.7 
76.0 
67.9 
76.8 
68.3 
79.9 

69.2 

77.9 

70.1 
79.9 

70.6 
79.2 
7i.2 
77.7 
7i.O 
81.8 
71.8 
77.6 

's.k 
73.6 
79.9 

75.7 
77.0 

79!3 

75.9 
80.5 
74.5 
79.3 
75.3 
80.6 
75. i 
80.0 


76.4 
81.5 
76.2 
80.3 
76.  i 
79.8 
75.7 
83.1 
76.0 
83.0 
76.1 
77.9 
76.5 
77.5 
76.3 
79.3 
76.7 
80.7 
76.9 
76.7 


79.1 

75.4 


78.3 
75.5 
78.1 
75.9 
82.8 
76.5 
80.9 
76.5 
81.3 
77.0 
83.6 
77.5 
85.3 
77.8 
84.7 
77.5 
81.5 
77.5 
85.6 
77.6 
83.3 
77.7 
82.6 
77.9 
80.3 
77.7 
79.4 
77.9 
81.1 
75.0 
83.2 
75.2 
83.1 
75.2 
84.0 
79.4 
84.6 
79.  i 
83.0 
79.1 
83.5 
79.0 
87.6 
79.4 
87.9 
79.5 
86.7 
79.5 
84.0 

85!o 
79.5 
83.5 
79.5 
83.1 
79.7 
82.9 
79.9 
83.3 
79.5 
82.6 
79.7 


81.4 
79.7 
80.3 

79.5 
79.0 

78.9 
80.6 
75.6 
80.5 
79.  i 
78.6 
75.7 
76.4 
75.0 
79.3 
75.2 
78.8 
75.2 
79.0 
75.  i 
81.6 
75.5 
81.3 
78.1 
84.7 
75.5 
83.5 

78.8 
75.2 
78.4 
75.0 
83.5 
75.2 
81.6 
75.1 
83.8 
75.4 
83.4 
75.5 
84.5 
75.4 
81.3 
75.5 
80.8 
75.4 
83.0 
75.2 
83.0 
75.2 
80.1 
75.0 
77.5 
77.6 
77.5 
77.5 
76.0 

76!9 

76!9 

77.1 

80.3 

75.5 


77.1 

76.8 
75.3 
76.7 
78.3 
76.6 
79.0 
76.6 
78.8 
76.5 
77.2 
76.7 
77.7 
76.4 
78.5 
76.5 
77.7 
76.4 
77.3 
76.  i 
73.3 
75.7 
73.3 
75.2 
73.3 
74.  i 
76.7 
7i.l 
76.4 
74. i 
78.0 
74.0 
79.1 
74.5 
73.8 
74.4 
76.4 
74.0 
76.8 
75.0 
73.7 
72.6 
68.8 
72.0 
70.6 
7i.6 
73.0 
71.1 
73.4 
70.9 
69.6 
70.6 
73.0 
70.5 
77.8 
71.0 
73.5 
70.5 
73.7 
70.0 


75.5 
74. ( 


70.7 
69.5 
66.5 
69.0 
66.4 
65.7 
68.1 
65.6 
68.6 
65.5 
67.1 
67.5 
67.7 
67.6 
66.8 
67.-? 
67.9 
67.5 
69.1 
67.5 
71.5 
67.2 
70.9 
66.5 
72.5 
66.5 
69.4 
66.3 
63.7 
66.0 
60.7 
65.2 
64.3 
64.6 
67.8 
65.4 
66.6 
65.0 
68.0 
64.6 
61.5 
64.2 
61.4 
63.0 
57.8 
62.9 
57.8 
62.5 
60.9 
62.6 
.59.4 
61.9 
59.5 
61.2 
59.7 
67.0 
60.1 
60.6 
68.9 
60.2 
58.6 
59.7 


65.i 


61.2 

59.2 
60.3 

55.7 
.^3.4 
57.7 
55.5 
57.4 
59.1 
57.1 
57.0 
.56.7 
53.0 
56.3 
.53.9 
.^1.3 
54.3 
54.9 
56.6 
5i.7 
59.7 
.54.5 
55.1 
54.5 
53.9 
53.6 
47.6 
,55.0 
48.4 
.'■>2.5 
47.8 
,5-?.  5 
63.6 
51.5 
48.7 
51.3 
47.9 
50.5 
48.9 
,'-)0.2 
49.7 
49.7 
.51.6 
49.5 
66.5 
49.5 
46.8 
49.2 
43.1 
45.4 
43.8 
47.4 
44.7 
47.0 
45.6 
46.5 
43.4 
46.5 
43.0 
46.5 


64.8     51.3    40.1 


52.4 


43.0 

/,6.2 
40.7 
45.5 
38. 4 
44.5 
40.3 
44.1 
42.9 
45.6 
44.2 
45.4 
38.4 
42.7 
41.3 
42.4 
43.7 
41.9 
45.5 
41.7 
44.5 
41.5 
41.6 
40.4 
43.0 
40.4 
43.6 
iO.2 
37.3 
.19.9 
34.7 
39.7 
36.3 
55.7 
38.0 
55.1 
34.8 
55.2 
32.2 
38.0 
43.0 
.S7.3 
43.9 
.S7.S 
38.3 
57.5 
40.8 
37.6 
36.7 
37.3 
33.7 
36.5 
36.4 
55.5 
39.3 
56.5 
40.2 
56.2 
39.8 
35.7 
46.9 


39.8 


Annual  average :  Air  60.3;  Water  57-1. 


Table  XXXV  shows  the  average  daily  temperature  of  the  surface  water  in 
the  harbor,  and  of  the  air,  at  2  p.  m.,  for  the  period  of  five  years  from  1882 
to  1886.  The  roman  figures  show  the  air  temperature,  and  the  italic  figures 
the  water  temperature. 


MARYLAND    WEATHER    SERVICE  147 

The  highest  annual  temperature  occurred  in  July  in  the  great  major- 
ity of  cases  in  the  past  34  years;  it  has  never  appeared  earlier  than 
June  3;  on  two  occasions  it  fell  in  the  months  of  September,  on  the 
7th  in  1881,  and  on  the  11th  in  1887.  The  average  interval  between 
the  occurrence  of  the  lowest  and  highest  temperatures  of  the  year  is 
181  days,  from  January  25  to  July  15.  The  longest  was  250  days,  from 
January  1  to  September  7,  1880;  the  shortest  was  116  days,  from 
February  10  to  June  6,  1899. 

The  details  of  occurrence,  together  with  the  minimum  and  maxi- 
mum temperatures  recorded,  are  shown  for  each  year  since  1871  in 
Table  XXXIV.  The  length  of  the  intervening  period  is  represented 
graphically  in  Fig.  37. 

Temperature  of  the  Water  in  the  Harbor. 

From  September  1,  1881  to  March  31,  1887,  observations  were  made 
daily  at  2  p.  m.  of  the  temperature  of  the  surface  water  in  the  harbor, 
from  the  wharf  at  the  foot  of  South  Street.  At  the  same  time  the  tem- 
perature at  the  bottom  (a  depth  varying  from  9  to  12  feet  according  to 
the  tide)  and  the  temperature  of  the  air  were  also  noted.  The  average 
values  of  the  surface  water  temperature  and  the  air  temperature  for  the 
five  years  from  1882  to  1886  are  presented  in  Table  XXXV.  Fig.  38 
also  shows  graphically  the  mean  temperature  of  the  water  at  the  surface, 
and  at  the  bottom,  and  of  the  air  at  2  p.  m.  The  temperature  of  the 
surface  water  is  approximately  5°  to  6°  cooler  than  the  air  temperature 
from  February  to  July.  The  difference  diminishes  gradually  from  July 
to  October.  In  October  the  temperatures  are  approximately  equal,  and 
i-emain  so  until  December  when  there  is  again  a  gradual  divergence.  The 
difference  between  the  temperature  of  the  surface  water  and  that  at  a 
depth  of  ten  feet  is  very  small  at  all  seasons  of  the  year,  averaging  about 
five-tenths  of  a  degree. 


11 


148  THE    CLIMATE    OF   BALTIMORE 


HUMIDITY. 


Introduction. — Two  distinct  gaseous  envelopes  surround  the  earth; 
one  is  the  dry  air,  with  small  quantities  of  other  relatively  permanent 
gases;  the  other  is  the  vapor  of  water  which  may  be  condensed  into 
visible  forms  of  dew,  frost,  cloud,  rain  or  snow,  under  ordinary  conditions 
of  temperature  and  pressure. 

It  is  of  the  highest  importance,  in  the  consideration  of  climatic  con- 
ditions, to  understand  the  functions  and  the  distribution  of  the  element 
of  water  in  the  atmosphere,  in  its  great  variety  of  forms  and  proportions. 
Water  is  being  changed  into  invisible  vapor  from  the  ice  and  snow  of  the 
frozen  north  no  less  constantly,  though  less  abundantly,  than  from  the 
warm  ocean  waters  of  the  tropics.  As  a  result,  the  atmosphere  is  never 
free  from  the  vapor  of  water.  It  may  be  present  in  small  quantities 
only,  as  in  the  dry  desert  regions  of  the  earth,  or  in  the  cold  zones  of  the 
north  and  south,  or  in  the  rare  and  cold  atmosphere  of  the  mountain 
tops.  The  vapor  capacity  of  a  given  space  increases  rapidly  with  in- 
crease in  temperature.  A  cubic  foot  of  vapor  at  a  temperature  of  50° 
F.  at  normal  sea-level  pressure,  and  at  saturation,  weighs  about  4  grains ; 
at  70°  it  weighs  8  grains;  and  at  100°,  about  20  grains;  hence  with  an 
increase  in  temperature  from  50°  to  70°  the  quantity  of  moisture  at 
saturation  is  doubled. 

The  invisible  vapor  of  water  in  the  atmosphere  is  generally  referred 
to  as  the  humidity  of  the  atmosphere.  When  the  amount  of  vapor  is 
actually  weighed  in  grains,  ounces,  or  pounds,  or  when  it  is  measured 
in  terms  of  pressure,  as  so  many  inches  of  mercury,  it  is  referred  to  as 
absolute  humidity.  When  it  is  measured  in  terms  of  percentage  of  the 
total  amount  which  can  exist  in  a  given  portion  of  the  atmosphere,  it  is 
referred  to  as  the  relative  humidity.  For  example,  as  stated  above,  the 
total  quantity  of  invisible  vapor  which  may  be  contained  at  ordinary 
pressure,  in  a  cubic  foot  at  70°  temperature,  is  8  grains.  The  atmosphere 
is  then  said  to  be  saturated,  and  the  relative  humidity  is  100  per  cent. 
Suppose  the  amount  of  vapor  to  be  reduced  to  4  grains,  or  one-half  the 
full  capacity,  the  temperature  remaining  the  same,  the  percentage  of 


MAEYLAND   WEATHER   SERVICE  149 

the  relative  hiimidity  would  then  be  50.     The  point  of  saturation  is  also 
called  the  dew  point. 

As  the  temperature  of  the  atmosphere  diminishes  rapidly  from  the 
earth's  surface  upward,  the  capacity  for  water  vapor  diminishes,  and  at 
a  more  rapid  rate.  Calculations  have  been  made  of  the  amounts  of 
aqueous  vapor  which  the  atmosphere  can  hold  in  suspension  at  different 
temperatures  and  below  given  altitudes.  In  the  following  table  by  Fer- 
rel,  the  figures  given  show  the  depth  in  inches  of  water  which  would  result 
if  all  of  the  vapor  which  it  is  possible  for  the  atmosphere  to  hold  in 
suspension  under  the  given  conditions  were  condensed  to  water. 

AMOUNT  OP  AQUEOUS  VAPOR  IN   SATURATED  ATMOSPHERE  OF   STATED 
TEMPERATURE  AND   DEPTH. 


Elevation. 

80°  F. 

70°  F. 

60°  F. 

50°  F. 

6,000 

feet. 

1.3  inch. 

1.0    lE 

ich. 

0.1   inch. 

0.5  inch. 

12,000 

2.1 

1.5 

1.1 

0.8 

18,000 

2.5 

1.8 

1.3 

0.9 

24,000 

2.7 

2.0 

1.4 

1.0 

30,000 

2.8 

2.1 

1.5 

1.1 

The  conditions  assumed  probably  never  occur  in  nature,  as  the  rapid 
decrease  in  temperature  with  elevation  would  preclude  the  possibility 
of  an  average  temperature  sufficiently  high,  or  at  unifonn  saturation, 
to  the  given  elevations.  Probably  under  the  most  favorable  conditions 
of  a  moist  atmosphere  in  the  tropics,  the  amount  of  vapor  from  the 
earth's  surface  to  the  upper  limits  of  the  atmosphere,  if  condensed,  would 
measure  less  than  two  inches. 

The  decrease  in  the  amount  of  vapor  in  the  atmosphere  with  decrease 
in  temperature  may  be  stated  in  another  form.  At  the  equator  the 
amount  of  vapor  may  reach  about  11  grains  per  cubic  foot,  or  about  20 
tons  per  cubic  mile;  at  latitude  of  40°  it  may  reach  about  5  grains  or 
about  9  tons  per  cubic  mile,  assuming  a  temperature  of  55°  as  an  average 
for  the  year;  at  latitude  70°  with  an  average  temperature  of  30°,  it  may 
attain  about  2  grains  per  cubic  foot,  or  about  3.5  tons  per  cubic  mile. 
It  has  been  estimated  that  one-half  of  the  total  quantity  of  vapor  in  the 
entire  atmosphere  lies  below  an  elevation  of  about  (>500  feet,  or  below  the 
summit  of  !Mt.  Washington ;  hence  the  decrease  in  quantity  is  very  rapid 


150  THE    CLIMATE    OF    BALTIMORE 

and  we  may  easily  realize  how  mountains  of  moderate  elevation,  and 
even  the  higher  hills,  may  afi'ect  the  rainfall  and  cloudiness  of  a  locality. 

As  the  absolute  humidity  of  the  atmosphere  at  any  given  place  depends 
upon  the  local  temperature  and  a  local  water  supply,  there  is  a  steady 
decrease  in  the  amount  of  water  vapor  from  the  equator  to  the  poles, 
from  the  surface  of  the  earth  upward,  and  from  the  oceans  toward  the 
interior  of  the  continents.  This  general  law  of  distribution  may,  how- 
ever, be  modified,  and  even  completely  reversed,  by  the  direction  and 
character  of  the  wind  movement  over  a  given  area.  While  the  vapor  of 
the  atmosphere  is  taken  up  by  evaporation  from  a  variety  of  surfaces, 
such  as  lakes,  rivers,  moist  fields,  or  the  foliage  of  the  forest,  the  great 
source  of  supply  must  always  be  the  surface  waters  of  the  equatorial  and 
tropical  oceans.  From  these  it  is  carried  up  by  the  winds  and  distrib- 
uted to  all  parts  of  the  globe. 

This  invisible  moisture  of  the  atmosphere  has  a  most  important  func- 
tion to  perform  in  tempering  the  heat  and  cold.  Where  it  is  found  in 
abundance,  extremes  of  temperature  are  unlikely.  Its  absorbing  power 
is  great  compared  with  that  of  dry  air,  and  the  earth's  surface  is  pro- 
tected against  the  heat  of  the  sun  by  day,  and  the  rapid  cooling  by 
radiation  during  the  night.  Its  abundance  in  the  tropics  is  largely 
responsible  for  maintaining  the  uniformly  high  temperature  of  those 
regions.  Its  absence  in  limited  areas  of  the  tropical  and  sub-tropical 
zones  is  marked  by  great  diurnal  fluctuations  in  temperature,  as  in  the 
arid  regions  of  the  southwest.  Most  important  of  all,  it  is  the  great 
source  of  supply  of  the  rainfall  and  snowfall  of  the  world;  without 
first  passing  into  the  form  of  vapor,  the  oceans  and  rivers  would  have 
but  little  effect  in  watering  the  fields  and  forests  of  the  earth. 

While  the  benefits  of  the  atmospheric  vapor  are  numerous  and  appar- 
ent, it  may  also  be  a  source  of  much  personal  discomfort.  When  the 
relative  humidity  is  high,  that  is,  when  the  vapor  is  near  the  saturation 
point,  or  dew  point,  evaporation  from  the  body  becomes  sluggish,  or 
ceases,  the  air  begins  to  feel  muggy,  when  the  temperature  is  high,  or 
raw  when  it  is  low.  A  temperature  which  would  be  considered  mod- 
erate with  a  dry  air  becomes  oppressively  hot  when  the  humidity  ap- 


MARYLAND   WEATHER   SERVICE 


151 


Mdt       3         6         9      Noon      3         6         9      Mot  Mdt.      3  6         9      Noon      3         6         9      Moi 


JAN. 

66 


^ 

' 

/ 

/ 

/ 

APRIL 

kJ 

MCT,  3 


9  Noon  3 


9  Mdt. 


— ~         -N 

/- 

YEAR 

-^ 

Mot        16         9      Njon      3  6  9      Mdt  Mdt       3  6  9      Noon      3  6         9      Mdt 


^ 

\ 

/ 

I 

j 

JULY 

J 

^ 

J 

1 

' 

OCT. 

J 

Fig.  39. — Mean  Hourly  Relative  Humidity. 

Till-  mean  hourly  relative  humidities  for  the  months  of  January,  April,  July,  October  and 
for  the  year  are  oxiircssed  as  pcn-entii^'os,  coniplctc  saturation  boiu},'-  reprcsouted  by  100  per 
cent.    The  curves  are  based  on  the  'M  months'  record  of  a  Richard  hytjrograph. 


153 


THE    CLIMATE    OF    BALTIMORE 


preaches  or  exceeds  80  per  cent.  On  the  other  hand,  a  cold  of  15°  or 
20°  above  zero  with  a  humidity  of  80  per  cent,  a  condition  which  is 
common  in  the  Atlantic  coast  states,  will  cause  much  suffering,  while 
temperatures  of  20°  below  zero  in  the  northwest,  with  a  humidity  of 
25  or  30  per  cent,  are  described  as  comfortable  and  exhilarating. 

The  atmosphere  does  not  lose  in  transparency  with  increasing  humidity 
either  relative  or  absolute;  on  the  contrary,  a  high  humidity  is  often 
accompanied  by  greater  clearness,  and  an  unusual  transparency  has 
come  to  be  regarded  as  a  sign  of  rain.  When,  however,  the  vapor  reaches 
the  dew  point,  just  beyond  the  point  of  saturation,  we  have  a  series  of 
phenomena  of  the  highest  interest  and  importance  to  us,  resulting  from 
condensation  into  the  visible  forms  of  dew,  fog,  clouds,  rain,  snow,  frost 
and  hail.  The  particular  form  of  condensation  is  primarily  a  function 
of  temperature,  modified  by  local  conditions  of  topography,  elevation 
above  the  earth's  surface,  and  the  movements  of  the  atmosphere. 

Hourly  Variations  in  Humidity. 


Continuous  automatic  records  of  the  variations  of  relative  humidity 
are  available  for  about  two  years  and  a  half  at  Baltimore.     While  this  is 
table  xxxvi.-mean  hourly  relative  humidity. 

(1902-1904.) 


1  A.  M. 

2 

3 

4 

5 

fi 


8. 


10 

11 

Noon... 

1  P.  M. 

2 

3 

4 

5 

.  6 


10 

11 

Midnight . 


Jan.    Feb.    Mar 


Averag-e. 


62 
61 
59 
58 
58 
60 
62 
64 
66 


er 


70 
71 
71 
72 
73 
74 
76 
75 
71 
70 
66 
64 
62 
60 
58 
59 
60 
62 
64 
66 
67 
68 
70 
70 


78 


.2     67.4     70.6     62.6 


Apr. 


76 


May  June  July  Aug.  Sept.  Oct. 


76 


80 
80 
77 
70 
66 
60 
55 
51 
50 
47 
46 
46 
48 
60 
54 
60 
64 
6S 
71 
73 
75 


.4  67.4  70.4  72.1  72.4 


Nov.  Dec 


72 
72 
72 
74 
74 
74 
74 
72 
66 
61 
56 
63 
50 
50 
51 
62 
66 
60 
62 
65 


70 
70 
70 
70 
70 
70 
70 
70 
66 
62 
69 
56 
64 
54 
55 
56 
69 
61 
63 
64 
66 


5  64.3  64.0  67.4 


An'l 


MARYLAND   WEATHER   SERVICE 


153 


a  much  shorter  record  than  that  utilized  in  the  discussion  of  tempera- 
ture, pressure,  wind  direction  and  other  factors,  it  is  still  of  sufficient 
length  to  enable  us  to  establish  firmly  the  form  of  the  diurnal  curve. 
The  diurnal  variation  in  the  relative  humidity  is  represented  by  a  simple 
curve  with  its  maximum  point  at  about  5  a.  m.  for  the  year,  but  varying 
between  4  a,  m.  and  7  a.  m.  according  to  the  season.  The  time  of  maxi- 
mum follows  closely  the  time  of  sunrise.  The  minimum  point,  or  the 
drj^est  time  of  day,  occurs  between  1.30  p.  m.  and  3.30  p.  m.     The 


70         s  H.  70 


Fig.  40.  Mean  Hourly  Relative  Humidity. 

As  in  Fig.  39,  the  hourly  humidities  are  expressed  as  percentages,  100  per  cent  repre- 
senting complete  saturation.  The  light  shades  represent  the  lower  humidities,  or  the 
dryer  portions  of  the  day  and  year ;  the  heavy  shades,  the  time  of  higher  humidities. 
The  dotted  lines,  S.  R.  and  S.S  indicate  the  time  of  sunrise  and  sunset  respectively. 
The  diagram  is  based  on  the  30  months'  record  of  a  Richard  hygrograph. 


details  of  the  hourly  variation  are  shown  statistically  in  Table  XXXVI, 
and  graphically  in  Fig.  39  and  Fig.  40.  The  seasonal  distribution  is 
revealed  at  a  glance  in  Fig.  40,  in  which  the  light  shades  represent  the 
lower  humidities,  or  the  dryer  portions  of  the  day,  and  increase  in  the 
density  of  the  shades  shows  an  increase  in  the  humidity.  It  will  be 
observed  that  the  dotted  line  representing  the  time  of  sunrise-  (S.  K.) 
passes  through  the  areas  of  heaviest  shading,  approximately  in  their 
central  portions.  The  values  in  both  tables  and  figures  are  shown  in 
percentages,  total  saturation  of  the  atmosphere  being  represented  by  100. 


154  THE    CLIMATE   OF   BALTIMORE 

The  actual  values  for  relative  humidity  from  hour  to  hour  during  the 
day  fluctuate  rapidly  on  days  with  unsettled  weather.  The  changes  are 
particularly  marked  during  the  course  of  a  thunderstorm.  There  is 
frequently  very  little  resemblance  between  the  curve  showing  actual  con- 
ditions and  that  representing  average  conditions  for  a  month  or  more. 
Every  change  in  the  direction  of  the  wind,  or  in  the  temperature,  is 
reflected  in  the  form  of  the  actual  curve.  In  Plate  VIII  some  typical 
relative  humidity  curves  made  by  the  self-recording  instrument  are  repro- 
duced and  they  may  thus  be  compared  with  the  curve  in  Fig.  39  repre- 
senting average  values. 

Direct  observations  of  relative  humidity  were  made  at  varying  hours 
of  the  day  from  time  to  time  in  past  years.  The  continuous  record 
enables  us  to  determine  the  corrections  to  be  applied  to  any  of  the  com- 
binations of  hours  employed  in  the  past  in  order  to  obtain  a  correct 
daily  mean  based  on  24  hourly  observations.  A  daily  mean  derived 
from  any  of  the  series  of  three  observations  per  day,  one  in  the  morning 
between  7  and  9,  one  at  3  or  4  in  the  afternoon  and  the  other  between 
9  and  11  at  night,  gives  a  value  so  nearly  equal  to  the  true  mean  based  on 
24  hourly  observations  that  the  corrections  to  be  applied  fall  within  the 
limits  of  probable  error  of  observation,  and  hence  may  be  neglected.  For 
the  series  of  observations  made  at  8  a.  m.  and  8  p.  m.  the  departures 
from  the  true  average  are  large  enough  to  require  the  application  of 
the  necessary  corrections  shown  in  the  following  figures : 

CORRECTIONS  TO  OBTAIN  TRUE  DAILY  MEAN  HUMIDITY. 

(Expressed  in  percentages.) 

Hours  of  Jan.   Feb.  Mar.  Apr.  May   June  Julj-  Aug-.  Sept.   Oct.   Nov.  Dec.  Year 

observation. 
(8a.m. +  8  p.m.)   -3.3°  -3.1°  -1.9°  -1.9°  -1.6°  -0.6°  -1.6°  -1.9°  -2.6°  -4.0°  -4.2°  -3.0°   -2.6° 
2 

Phases  of  the  Diurnal  March  of  Eelative  Humidity. 

The  time  of  occurrence  of  the  maximum  and  minimum  values  for  the 
day,  and  the  time,  in  hours  and  minutes,  when  the  humidity  is  the  same 
as  the  mean  for  the  entire  day,  are  sho^^^l  in  the  following  table  and  in 
Fig.  41 : 


MARYLAND   WEATHER    SERVICE  155 

PHASES   OF  THE    DIURNAL  MARCH   OF   RELATIVE   HUMIDITY. 


S      s    ^,S 


§ 

>> 

3 

3 

1-5 

>-s 

OQ        O 


O       o       a> 


Max.  (a.m.) 

First  mean  (a.  m.)  — 

Min.  (p.m.) 

Second  mean  (p.  m.). 


7.30  7.20  5.40  4.30  4.30  4.00  4.30  5.30  5.00  4.30  5.30  4.30  5.00 

10.00  10.40  9.20  8.50  8.30  8.30  8.20  8.40  8.40  8.50  9.20  9.30  9.30 

2  30  3.20  3..30  3. 00  2. 30  2. 00  3. 00  2. 30  1.30  2. 30  1.30  1.30  2. 80 

7'.00  9.00  8.40  8.40  7.50  8.20  7.50  8.00  7.20  7.20  7.40  8.00  7.60 


JFMAMJJASONDJ 


6   A.  M. 


/ 

"^ 

\, 

^ 

A 

% 

r 

"^ 

.^ 

, 

B 

/ 

'^ 

s.^^ 

^ 

"^ 

\^ 

y 

N^ 

y 

V 

y 

•^ 

— ' 

^ 

^ 

^ 

V 

S 

^ 

— 1 

■ — 

C 

, 

■ 

, 

^. 

I 

N 

N, 

i 

\ 

/^ 

-^^ 

,^ 

^V 

' 

^^ 

0^ 

y 

'^ 

V 

/ 

6     P.  M. 


Fig.  41. — Phases  of  the  Diurnal  Variations  in  Relative  Humidity. 

B  shows  the  variations  in  the  hour  of  occurrence  of  the  dryest  time  of  the  day  from 
month  to  month  ;  D  the  time  of  occurrence  of  the  dampest  portion  of  the  day  ;  A  and  C 
show,  respectively,  the  afternoon  and  the  morning  hours  when  the  mean  humidity  of 
the  day  is  most  likely  to  occur. 


156 


THE    CLIMATE    OF    BALTIMORE 


Mean  Monthly  and  Annual  Eelative  Humidity. 
The  observed  values  of  the  relative  humidity  for  the  entire  period 
from  1871  to  1903  were  reduced  to  true  mean  monthly  and  annual  values 
based  upon  hourly  observations;    these  corrected  figures  are  contained 


i          F         M         fi 

^ 

J 

A       s      o       ^ 

D         J 

70 

N 

N, 

/^ 

^-^ 

/ 

■\ 

V 

^ 

/ 

60 

V 

y 

r — 

50 

7Q 


50 


Fig.  42. — The  Mean  Monthly  Relative  Humidity. 


The  diagram  is  based  on  direct  observations  at  two  or  three  stated  periods  of  the 
day  during  a  period  of  30  years;  the  average  values  were  corrected  for  the  diurnal 
variation,  and  expressed  as  percentages  of  total  saturation. 


IB/S                             1880                             1885                            lo 

»0                               1895                             1900 

V»._^ 

/\ 

f\y\ 

/■\ 

'^Vy 

V 

Fig.  43. — Variations  in  the  Mean  Annual  Relative  Humidity. 
(Expressed  as  percentages  of  total  saturation.) 

in  Table  XXXVII,  together  with  the  ten-year  monthly  averages  and  the 
normals  for  the  entire  period.  The  monthly  normals  and  the  variations 
in  annual  means  are  also  graphically  shown  in  Fig.  43  and  Fig.  43 
respectively. 


jVIAEYLand  weather  service 


157 


The  normal  amount  of  moisture  in  the  atmosphere  for  the  entire  year 
is  about  two-thirds  of  the  total  capacity  of  the  atmosphere  for  water 
vapor,  namely,  66.5  per  cent.  The  mean  monthly  amounts  vary  from 
season  to  season,  being  greatest  in  the  month  of  September  (70.4)  and 

TABLE  XXXVII.— MEAN  MONTHLY  RELATIVE  HUMIDITY. 


Year 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

An'l 

18T1 

68 
63 
66 
75 
73 

68 
74 
72 
71 
73 

71 
74 
74 
67 
65 

76 
69 
67 
67 
66 

67 
76 
72 
69 
70 

63 
66 
70 
66 
71 

69 
70 
66 

66 

68 
60 
73 
68 

66 
66 
68 
70 
66 

66 
66 
66 
68 
63 

69 
71 
68 
61 
70 

69 
75 
69 
67 
68 

66 
71 

63 

73 
68 

69 

73 
62 

82 
68 
64 
66 
68 

66 
70 
64 
64 
63 

63 
63 
66 
66 
66 

63 
60 
64 
61 
64 

70 
71 
65 
61 
60 

69 
63 
67 

72 
66 

73 
76 
73 

66 
50 
59 
66 
60 

56 
66 
64 
57 
56 

69 
60 
65 
57 
56 

69 
69 
51 
63 

60 

65 
63 

68 
68 
63 

62 
65 
57 
61 
69 

63 

68 
54 

66 
60 
65 
69 
51 

64 
64 
63 

58 
68 

66 
70 
59 
59 
65 

71 

68 
68 
66 

67 

59 
68 
66 
67 
66 

66 
60 
68 
63 
69 

72 
64 
66 

68 
60 
64 
64 
63 

66 
67 
64 
64 
63 

69 
60 
68 
66 
63 

71 
66 
64 
73 
64 

71 
76 
73 
66 
69 

69 
64 
61 
67. 
71 

68 
66 
75 

65 
60 
63 
63 
65 

63 
68 
66 
61 
64 

61 
66 
67 
65 
63 

73 
71 
66 
73 
62 

77 
71 
65 
64 
64 

68 
70 
66 
68 
63 

74 
73 
65 

69 
65 
78 
62 
76 

69 

62 
73 
70 
71 

60 
74 
63 
69 
69 

71 
70 

68 
70 
70 

78 
73 
65 
68 
64 

63 
68 
73 
70 
68 

76 
69 

76 

65 

66 
76 

72 
66 

73 
71 

73 
70 
67 

68 
74 
70 
63 
67 

70 
70 

74 
75 
74 

78 
69 
67 
73 
67 

66 
65 
65 
69 
83 

74 
75 
70 

66 
62 

73 
66 
67 

66 
71 
66 
68 
67 

67 
76 
73 
64 
76 

66 
64 
65 
64 
68 

71 
63 
71 

67 
63 

63 
70 
69 
71 
80 

66 
74 
63 

65 
67 
69 
63 
66 

70 
68 
67 
63 
66 

67 
65 
64 
61 
69 

61 
60 
67 
71 
63 

70 
71 
68 
60 
70 

65 
66 
67 
67 

72 

66 

73 
66 

63 

58 
71 
66 
74 

71 
66 
68 
69 
69 

70 
65 
67 
68 
62 

71 

68 
62 
67 
62 

67 
71 
68 
68 
68 

63 
72 
66 
64 

73 

77 
70 
57. 

67.4 

1872 

58.8 

1873 

66.1 

1874 

66.4 

1^75       .  ... 

66.3 

1876 

1877 

66.3 

67.7 

1878 

67.2 

1879 

66.4 

1880 

1881 

1882 

1883 

65.0 

65.6 
67.7 
65.9 

1884 

64.4 

1885 

64.3 

1886 

1887 

1888 

1889 

69.0 
66.3 
65.3 
67.5 

1890 

1891 

1893 

1893 

66.8 

69.3 
70.5 
68.1 

1894 

65.7 

1895 

64.2 

1896 

64.3 

1897 

65.8 

1898 

1899  .... 

65.8 
67.5 

1900  

69.3 

1901 

1902 

69.7 
70.1 
65.2 

Average,  1871-80 

1881-90 

1891-1900 

70.2 
69.5 

ra.o 

66.9 

66.8 
67.6 

66.3 
61.4 
65.4 

59.8 
69.9 
60.0 

59.8 
65.9 
64.3 

64.3 
66.3 
68.7 

63.8 
66.6 
67.5 

69.3 

68.4 
68.8 

69.7 
70.5 
70.3 

67.1 
68.0 
67.6 

65.4 
64.7 
67.6 

67.5 
66.3 

67.8 

65.7 
66.2 
67.0 

1871-1903 

69.6 

66.6 

64.9 

eo.i 

63.6 

66.6 

66.4 

69.3 

70.4 

67.6 

65.8 

67.2 

66.6 

least  in  the  month  of  April  (60.1).  For  individual  years,  the  monthly 
averages  vary  considerably.  For  example,  the  average  humidity  for  the 
month  of  March  has  been  as  high  as  82  per  cent  and  as  low  as  54  per 
cent,  a  range  of  28.  A  similar  range  has  been  experienced  in  the  month 
of  October.     The  month  possessing  the  smallest  range  in  the  monthly 


158  THE    CLIMATE    OF   BALTIMORE 

average  value  is  January,  with  a  maximum  of  76  per  cent  and  a  minimum 
of  62  per  cent,  a  range  of  14. 

The  range  in  actual  conditions  of  humidity,  as  distinguished  from 
average  conditions  for  a  considerable  period,  shows,  of  course,  much 
greater  fluctuations.  As  the  upper  limit,  namely,  100  per  cent,  is  reached 
at  all  seasons  of  the  year  during  misty  or  rainy  weather,  the  lower  limit 
is  the  index  of  variability.  The  lowest  values  are  most  likely  to  occur 
during  the  clear,  cold  days  of  winter  or  early  spring.  During  the  two 
years  and  a  half  in  which  continuous  automatic  registration  was  main- 
tained at  Baltimore,  the  humidity  during  the  afternoon  hours  occasion- 
ally fell  to  25  per  cent,  or  one-fourth  the  moisture  capacity.  The  occa- 
sions upon  which  the  moisture  content  fell  below  this  percentage  were 
rare.  A  few  of  the  exceptionally  dry  days  during  this  period  are  here 
cited : 

EXCEPTIONALLY  DRY  DAYS. 
January  31,  1903,  minimum  humidity  was  20  per  cent. 


April  5,   1904, 

"     11    ••       • 

May  4,   1904, 

"     15    " 

August  17,   1902, 

"     24    "       ' 

October  30,   1903, 

"     14    " 

November   9,    1902, 

"     22    " 

The  limits  of  variability  in  the  annual  average  humidity  during  33 
years  were  70.5  per  cent  in  1892,  and  58.8  per  cent  in  1872,  a  range  of 
11.7  per  cent.  The  ten-year  averages  have  only  varied  from  the  normal 
for  the  entire  period  by  the  following  small  departures: 

1871  to  1880 0.8  below  normal ; 

1881  to  1890 0.3  below  normal ; 

1891  to  1900 0.5  above  normal. 

Absolute  Humidity. 

Expressing  the  humidity  in  the  terms  of  the  actual  weight  of  the  water 
vapor  in  the  atmosphere  at  different  hours  of  the  day  throughout  the 
year,  the  distribution  is  shown  in  the  following  table.  These  figures 
show  the  average  amount  of  water  present  in  the  atmosphere  at  the  hours 
specified  during  the  five  years  from  August,  1881,  to  July,  1886.  As 
the  amount  of  moisture  in  the  atmosphere  is  primarily  a  function  of 
the  temperature  of  the  air,  we  find  a  steady  increase  in  the  absolute 


SELECTED   RELATIVE    HUMIDITY    CURVES. 


MARYLAND    WEATHER   SERVICE 


159 


humidity  from  January,  the  coldest  month,  to  July,  the  warmest  month 
of  the  year. 

MEAN  ABSOLUTE  HUMIDITY. 
(Weight  of  the  vapor  of  water  in  grains  per  cubic  foot.) 


Hours  of 
Observation 


1.41 
1.47 
1.60 
1.6" 
11  p.m 1.54 


7  a.  m 

11  a.  m  . 

3  p.  m  . 

7  p.m 


Jan. 


Feb. 


1.58 
1.66 
1.71 
1.72 
1.72 


aiar. 


Average 1.619  1 


1.78 
1.78 
1.84 
1.91 
1.82 


Apr. 


2  72 
2171 
2.78 
3.01 
2.93 


1.827   2.828  3.997   5 


May 


3.89 

3.80 
4.02 
4.20 
4.08 


June 


5.56 
5.50 
5.51 
5.79 

5.82 


July  Aug.  Sept. 


6.50 
6.12 
6.11 
6.68 
6.71 


5.98 
6.06 
6.16 
6.38 
6.38 


5.29 
5.53 

5.48 
5.79 
5.61 


Oct.    Nov.  Dec. 


3.83 

3.87 
4.06 
3.96 
4.03 


6.425   6.193   5.539   3.951    2.411    1.836   3.267 


2.26 
2.36 


2. 47 

2.48 


1.74 
1.82 
1.91 

1,88 
1.83 


Year 


3.082 
3.222 
3^267 
3.437 
3.327 


Mean  Vapor  Pressure. 
The  average  monthly  values  of  the  tension  of  the  vapor  of  water  in 
the  atmosphere,  based  upon  observations  of  temperature  of  the  air  and 
the  temperature  of  the  wet-bulb  thermometer  at  8  a.  m.  and  8  p.  m. 
from  1892  to  1903,  are  shown  in  the  following  table  expressed  in  frac- 
tions of  an  inch  of  mercury: 


Jan.     Feb. 

Mar. 

( 
April    May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 
.152 

Year 

.136       .133 

.193 

.252 

.394 

.549 

.631 

.614 

.512 

.334 

.222 

.344 

PRECIPITATION. 
Introduction, 

The  German  meteorologist.  Dove,  has  aptly  compared  the  atmosphere 
to  a  huge  still,  of  which  the  sun  is  the  furnace,  and  the  sea  the  boiler, 
while  the  cold  air  of  the  higher  elevations  and  of  the  temperate  zones 
plays  the  part  of  the  condenser;  we,  on  a  wet  day,  catch  some  of  the 
liquid  which  distills  over. 

The  condition  and  method  of  formation  of  dew,  fog,  cloud,  rain,  snow 
and  hail  are  admirably  and  concisely  stated  in  the  following  extract  from 
one  of  Dr.  Hann's  recent  treatises :  * 

'  Hann,  J.  Allgemeine  Erdkunde.  I.  Abtheilung.  8°  Wien,  1896.  pp.  173 
et  seq. 


160  THE    CLIMATE   OF   BALTIMORE 

"  Condensation  of  moisture  gives  rise  to  numerous  phenomena  which 
are  collectively  called  liydrometeors.  It  takes  place  whenever  the  tem- 
perature falls  below  the  dew  point.  Hence  whatever  favors  a  lowering 
of  the  temperature  of  the  air  favors  the  production  of  dew,  fog,  cloud, 
rain,  snow  and  hail. 

"  The  ground  cooling  rapidly  during  a  clear,  calm  night  by  radiation, 
lowers  the  temperature  of  the  air  resting  upon  it;  we  then  have  dew. 
With  a  temperature  below  freezing  we  will  have  frost.  Warm,  moist 
air  mixing  with  colder  air  near  the  earth's  surface  will  give  us  fog; 
thus  we  see  fog  formed  over  rivers  in  the  early  morning.  Over  the  Banks 
of  New  Foundland  we  have  a  cold  body  of  water  over  which  pass  warm, 
moist  southerly  winds,  producing  almost  continual  fog.  With  a  tem- 
perature below  freezing  point  the  fog  collects  on  trees  and  shrubs  in 
the  form  of  hoar  frost.  Mixing  of  air  currents  at  different  temperatures 
high  above  the  earth,  and  rising  and  cooling  moist  air,  produce  clouds. 
Fog  and  clouds  are  composed  of  small  drops  of  water.  In  winter  and 
at  high  altitudes  they  are  composed  of  ice  crystals.  High  cirrus  clouds 
all  the  year  round  are  composed  of  ice  crystals.  At  an  elevation  of  3500 
meters  (11,483  feet)  in  the  middle  latitudes  the  air  temperature  is 
below  the  freezing  point  throughout  the  year.  The  presence  of  ice 
crystals  in  the  higher  clouds  is  shown  by  the  colored  rings  about  the 
sun  and  moon.  These  ice  crystals  and  drops  of  water  float  in  the  air 
because  of  the  great  amount  of  surface  exposed  to  the  resistance  of  the 
air  in  comparison  with  their  weight. 

"As  fog  and  cloud  increase  in  density  the  drops  coalesce  and  become 
larger,  until  they  become  too  large  and  heavy  to  be  supported  by  the 
resistance  of  the  air;  they  then  go  over  into  rain.  Should  the  rain  pass 
through  drier  strata  of  atmosphere  it  may  be  reabsorbed.  In  winter 
condensed  water  crystallizes  and  falls  as  snow.  In  stormy  weather  and 
a  temperature  near  freezing,  the  snow  packs  and  balls  and  falls  as  snow 
pellets.  As  these  become  firmer  and  pass  alternately  through  layers  of 
air  above  and  below  freezing  point  they  become  coated  with  water  in  the 
warmer  layer  and  with  ice  in  the  colder  layer,  and  thus  form  hail. 

"  With  an  increasing  quantity  of  vapor  in  the  atmosphere  the  entire 


MARYLAND   WEATHER    SERVICE  161 

heavens  become  of  a  whitish  dimness  and  lunar  or  solar  halos  appear.  In 
most  cases,  however,  the  condensed  vapor  is  not  uniformly  distributed 
in  the  atmosphere,  but  is  collected  in  masses  which  float  in  the  air,  re- 
flecting the  light  and  throwing  shadows.     We  then  call  them  clouds." 

The  Causes  of  Precipitation. 

The  theory  that  rainfall  is  primarily  a  result  of  the  mixing  of  moist 
air  currents  at  different  temperatures  has  lost  much  in  faVbr  among 
meteorologists  of  to-day.  Calculations  have  shown  that  the  rain  result- 
ing from  such  a  cause  must  be  comparatively  light,  and  would  not  at 
all  account  for  the  abundant  precipitation  of  the  tropics,  or  the  heavy 
falls  connected  with  the  movement  of  storms.  Clouds  may  undoubtedly 
be  accounted  for  upon  the  supposition  of  a  mixture  of  air  currents,  with 
high  humidity,  especially  the  stratified  forms,  but  even  here  it  is  neces- 
sary to  find  a  more  abundant  source  of  condensation.  This  is  found  in 
the  agency  of  a  rising  current  of  air.  As  already  stated,  the  atmosphere 
contains  at  all  times  a  considerable  amount  of  moisture,  especially  in 
the  lower  layers  of  the  warmer  climates.  Conditions  are  favorable  for 
an  upward  movement  of  the  air  wherever  there  are  opposing  currents  or 
where  there  is  a  considerable  difference  in  temperature  over  adjacent 
areas.  Such  conditions  are  always  present  within  storm  areas,  or,  on  a 
larger  scale,  in  the  equatorial  region  where  we  have  the  northeast  and 
southeast  trades  meeting  and  causing  a  slow  upward  movement  of  the 
atmosphere  at  all  times.  Eising  currents  with  the  attendant  decrease  in 
temperature,  combined  with  the  presence  of  moisture,  are  capable  of 
accounting  for  the  heaviest  rainfalls  recorded. 

The  Geographical  Distribution  of  Rainfall. 

The  rainfall  of  a  locality  being  primarily  dependent  upon  the -quantity 
of  moisture  in  the  atmosphere,  that  is,  the  absolute  humidity,  and  this 
being  in  turn  dependent  upon  the  temperature,  we  find  a  steady  decrease 
in  the-  amount  of  precipitation  from  the  equator  to  the  poles.  In  the 
equatorial  regions  the  annual  rainfall  averages  about  75  inches,  while 
within  the  Arctic  circle  it  is  reduced  to  less  than  10  inches.     The  decrease 


163  THE    CLIMATE    OP    BALTIMORE 

is  not  at  all  uniform,  as  other  factors  enter  into  the  problem  of  distri- 
bution to  such  an  extent  as  to  overbalance  the  effect  of  the  temperature. 
The  prevailing  distribution  of  atmospheric  pressure,  the  prevailing  wind 
direction,  and  topography,  may  separately  or  in  combination  be  the 
determining  factors  in  the  distribution  of  rainfall  over  any  given  area. 

The  Influence  of  Wind  Direction. 

The  effect  of  wind  direction  may  readily  be  inferred  from  what  has 
already  been  stated  concerning  the  sources  of  atmospheric  moisture.  The 
oceans  being  the  great  source  of  supply,  and  the  winds  being  the  carriers 
and  distributors  of  moisture,  it  follows  that  rainfall  is  most  copious  along 
the  coasts,  and  decreases  with  distance  from  the  coasts  when  the  wind 
direction  is  from  the  oceans  inland.  In  the  United  States  there  is  a 
steady  decrease  from  the  Gulf  and  Atlantic  coasts  toward  the  central 
portions  of  the  continent,  being  50  inches  to  60  inches  in  the  southeast, 
and  about  15  inches  in  the  Eocky  Mountain  region.  On  the  Pacific  coast 
a  similar  decrease  obtains  but  is  here  greatly  modified  by  the  presence  of 
high  mountain  ranges  near  the  coast.  The  same  is  true  in  a  general 
way  over  all  of  the  continental  areas. 

The  Influence  of  Topography. 

The  distribution  of  rainfall  along  the  Pacific  coast  from  California 
northward  affords  an  excellent  illustration  of  the  influence  of  mountain 
ranges  crossing  the  path  of  rain-bearing  winds.  During  the  rainy  season 
the  moist  and  comparatively  warm  winds  from  the  Pacific  are  first  forced 
up  over  the  Coast  Eange  and  again  over  the  Sierra  Nevadas  to  altitudes 
varying  from  a  few  thousand  to  over  ten  thousand  feet.  The  moisture  is 
cooled  below  the  saturation  point  and  rains  are  copious.  The  winds 
descend  upon  the  eastern  slope  of  the  Sierra  Nevada  Mountains  com- 
jjaratively  dry.  While  the  annual  rainfall  on  the  Pacific  coast  decreases 
from  about  50  inches  to  10  inches  in  passing  over  a  distance  of  about  two 
hundred  miles  inland,  a  similar  decrease  from  the  Atlantic  and  Gulf 
coasts  extends  over  1500  to  2000  miles. 

In  northern  India  the  elevated  range  of  the  Himalaya  Mountains  lies 


MARYLAND   WEATHER   SERVICE  163 

directly  in  the  path  of  the  southwest  monsoon  winds.  The  winds,  blow- 
ing for  days  over  the  warm  waters  of  the  Indian  Ocean,  reach  the  moun- 
tains heavily  laden  with  moisture,  and  are  forced  up  the  southern  slope 
to  elevations  of  20,000  feet  and  more  before  they  can  proceed  further  on 
their  way  to  the  deep  and  persistent  barometric  depression  which  is  cen- 
tered to  the  north  of  the  mountains  during  the  warm  season.  Among 
the  foothills  on  the  southern  slope  of  the  mountains,  just  north  of  Cal- 
cutta, and  at  an  elevation  of  about  4500  feet,  lies  the  station  of  Cherra 
Poongee,  which  has  the  heaviest  annual  rainfall  in  the  world.  The  aver- 
age annual  fall  for  25  years  approximates  475  inches;  the  annual  amount 
has  varied  from  about  300  inches  to  over  900  inches,  nearly  all  of  which 
falls  in  the  six  months  from  April  to  September. 

The  Influence  of  Atmospheric  Pressure. 

As  previously  pointed  out,  conditions  which  favor  the  production  of 
rising  air  currents  are  favorable  to  the  production  of  rain.  Areas  over 
which  the  barometer  is  relatively  high  are  apt  to  be  poor  in  rainfall, 
and  areas  with  a  low  barometer  in  comparison  with  adjacent  areas  are 
apt  to  be  comparatively  rich  in  rainfall,  other  conditions  being  equal. 
This  broad  generalization  may  be  verified  by  almost  any  daily  weather 
chart  which  may  be  consulted,  and  is  familiar  to  all  who  have  occasion  to 
study  the  weather  charts  issued  by  the  United  States  Weather  Bureau, 
or  those  of  any  other  nation  issuing  such  charts.  When  we  see  upon 
these  charts  an  enclosed  area  of  low  barometer,  or  a  "Low,"  as  it  is 
familiarly  called,  cloudiness  and  rain  prevail  within  this  area;  where  the 
barometer  is  high,  relatively,  the  skies  are  prevailingly  clear  or  partly 
clouded.  "  High  area  "  weather  is  proverbially  "  fine  "  weather ;  "  low 
area  "  weather  is  generally  "  bad  "  weather.  As  the  winds  flow  toward 
the  center  of  an  area  of  low  barometer  from  all  sides,  the  air  at  and  near 
the  center  must  necessarily  rise,  and  rising,  it  is  cooled.  If  it  rises  high 
enough,  cloud  formation  and  rain  follow.  On  the  other  hand,  where 
the  barometer  is  relatively  high,  the  air  descends  and  the  winds  blow  out 
from  the  central  portions  of  the  high  area  in  all  directions.  We  have 
seen  that  ascending  air  is  cooled  by  expansion  and  radiation  as  it  rises; 
12 


164  THE    CLIMATE    OF   BALTIMORE 

conversely,  descending  air  is  warmed  by  compression.  As  it  warms,  its 
capacity  for  moisture  increases,  and  not  having  an  opportunity  to  take 
up  more  moisture  in  its  descent,  it  becomes  relatively  drier;  its  relative 
humidity  is  decreased.  Clouds  which  may  be  within  this  area  at  the 
beginning  of  its  formation  tend  to  dissolve,  and  near  the  center  where 
the  descent  of  air  is  most  active,  the  skies  are  apt  to  be  clear.  These 
areas  of  low  and  high  pressure  (cyclones  and  anticyclones  as  they  are 
technically  termed)  move  from  west  to  east  in  rapid  succession  in  the 
middle  latitudes  and  constitute  the  distinguishing  feature  of  our  weather. 
In  equatorial  regions  there  is  a  belt  of  varying  width  in  which  the 
pressure  is  constantly  lower  than  it  is  to  the  north  or  south.  Within 
this  belt,  situated  between  the  northeast  and  southeast  trade  winds,  the 
air  has,  in  addition  to  its  westward  drift,  an  upward  movement,  produc- 
ing the  "cloud  belt"  or  doldrums  with  its  almost  daily  copious  showers. 
To  the  north  of  the  northeast  trades  and  south  of  the  southeast  trades 
there  are  broad  belts,  most  regularly  developed  over  the  southern  hemi- 
sphere where  the  surface  conditions  are  more  uniform,  in  which  the 
pressure  is  relatively  high.  Here  the  air  has  a  descending  tendency  and 
these  areas  are  characteristically  dry.  Within  them  are  the  great  desert 
regions  of  the  globe — the  Sahara,  the  Arabian  desert,  the  arid  regions  of 
Australia,  as  well  as  those  of  our  own  Southwest.  Over  the  oceans  they 
are  known  as  the  "  horse  latitudes "  with  their  light  winds  and  scanty 
rainfall. 

The  Seasonable  Distribution  of  Eainfall. 

Climates  are  often  classified  according  to  the  manner  in  which  the 
rainfall  is  distributed  through  the  year.  We  have  regions  of  perennial 
rainfall,  as  in  the  United  States  east  of  the  Mississippi  Eiver,  where  there 
is  a  fairly  uniform  precipitation  throughout  the  year.  This  is  a  condi- 
tion which  prevails  with  limited  exceptions  between  latitudes  35°  and 
60°.  Within  the  tropics,  and  over  limited  areas  elsewhere,  as  in  the 
upper  Missouri  Valley,  there  are  large  areas  in  which  most  of  the  annual 
fall  of  rain  occurs  in  the  summer  months,  with  light  rain  in  winter  and 
spring.  In  other  regions,  as  in  the  Pacific  coast  states,  nearly  all  of  the 
precipitation  occurs  in  the  winter  months  with  little  or  no  rain  in  sum- 


MARYLAND   WEATHER   SERVICE 


165 


mer.     Lastly  there  are  the  arid  regions  of  the  world  which  are  nearly 
free  from  rain  throughout  the  year. 

Hourly  Amount  of  Eaixfall. 

A  diurnal  period  in  the  relative  amounts  and  frequency  of  rainfall  is 
most  distinctly  revealed  in  tropical  countries,  but  is  still  clearly  shown 
in  the  summer  months  of  the  middle  latitudes.  The  precipitation  which 
occurs  in  connection  with  the  movement  ot  a  general  barometric  depres- 
sion has  a  fairly  uniform  distribution  throughout  the  day;    that  occur- 


FiG.  44. — Average  Hourly  Precipitation. 

The  average  amount  of  rainfall  or  snowfall  during  each  hour  of  the  day,  for  every 
month  of  the  year,  is  shown  by  the  heavy  black  lines  and  the  shaded  areas.  The  light 
shades  show  the  time  of  day  and  year  when  the  precipitation  is  usually  lightest.  The 
figures  attached  to  the  curved  lines  show  the  amount  of  the  precipitation  In  hundredths 
of  an  inch.  The  values  are  based  on  the  ten  years'  record  of  a  tipping-bucket  rain- 
gage  and  on  eye  observations.  Only  days  with  an  appreciable  amount  of  precipitation 
were  considered  In  determining  the  average  amount  of  precipitation  for  the  day. 

ring  in  connection  with  thunderstorms  is  restricted  mostly  to  the  after- 
noon hours,  and  is  intimately  associated  with  the  diurnal  variation  in 
temperature  and  pressure. 

The  most  conspicuous  feature  of  the  diagram  representing  the  hourly 
quantity  of  rainfall  (see  Fig.  44)  is  the  uniform  distribution  of  the  pre- 
cipitation throughout  the  day  during  the  winter  and  spring  months. 
This  is  doubtless  explained  by  the  fact  that  the  winter  and  spring  snows 
and  rains  occur  in  connection  with  the  more  or  less  regular  succession  of 


166 


THE    CLIMATE    OF   BALTIMORE 


the  cyclonic  disturbances  of  the  middle  latitudes  whose  eastward  progress 
is  but  slightl}',  if  at  all,  affected  by  the  diurnal  variations  of  temperature 
and  pressure.  On  the  other  hand,  the  intensity  of  summer  rains  has  a 
distinct  diurnal  period,  being  light  in  the  forenoon,  increasing  rapidly  to 
noon,   or    1    p.    m.,    and    then   more   slowly   to    a   maximum    at   about 


TABLE  XXXVIII.— TOTAL  HOURLY  RAINFALL  PER  MONTH  AND  YEAR. 
(In  hundredths  of  an  inch.) 


Hours. 

i 

05 

p. 

^ 

0) 

a 

3 

^       bi 

3    1    3 

0 

+3 

0 

> 
0 

oil 

S 

3 

n 

.16 

.16 

< 

S 

>-i 

>-> 

.09 

<! 

cc 

0 
.15 

12; 

.17 

Q 
.11 

03 

1.70 

§ 

Md't. 

to  1  A.M 

.11 

.11   .11    .OS 

.35  .10 

.14 

1 

tk         0              41 

.09 

.13  .18'  .09:  .18,  .06 

.03   .12   .06 

.15 

.15 

.14 

1.38 

.12 

2 

"    3      " 

.09 

AV  .16   .15!  .16   .05 

.13|  .10   .09 

.15 

.13 

.09 

1.40 

.12 

3 

"    4      " 

.111  .11    .13;  .15   .12   .06 

.OS;   .02    .16 

.14 

.12 

.12 

1.32 

.11 

4 

"     5      " 

.06   .13   .13   .20   .11    .06 

.03    .06   .05 

.09 

.10 

.08 

1.10 

.09 

5 

"     6      " 

.Os!  .16'  .13  .17i  .12!  .06 

.04    .07    .09 

.16 

.09 

.07 

1.24 

.10 

6 

"    7      " 

.07i  .14   .15   .13  .12   .10 

.05    .10    .11 

.09 

.12 

.11 

1.29 

.11 

7 

"    8      " 

.08   .15   .12   .17J  .09   .04 

.04    .08   .06 

.20 

.08 

.15 

1.26 

.10 

8 

"    9      " 

.07i  .18   .13  .14    .08    .04 

.04    .04   .04 

.10 

.07 

.14 

1.07 

.09 

9 

"  10      " 

.10    .22|  .13,  .14,  .10    .02 

.08    .02   .08 

.11 

.10 

.15 

1.25 

.10 

10 

"  11      " 

.13   .171  .111   .12    .07    .05 

.06'  .08    .08 

.20 

.08 

.16 

1.31 

.11 

11 

"  Noon 

.12    .14    .121   .10    .0.5    .05 

.04    .28    .08 

.15 

.07 

.15 

1.35 

.11 

Noon 

"    1  P.  M 

.15   .14'  .10   .11   .13   .08 

.21    .17    .15 

.14 

.11 

.15 

1.64 

.14 

1 

"    3      " 

.16    .14'  .12!  .14'  .19    .10 

.16 

.13  .17 

.10 

.10 

.14 

1.66 

.14 

3 

"    3      " 

.121  .12    .13   .12,  .16 

.23 

.36 

.18  .24 

.08 

.10 

.14 

1.98 

.16 

3 

"    4      " 

.11    .13    .19    .15    .12 

.22 

03 

.26   .27   .09 

.18 

.14 

2.08 

.17 

4 

"    5      " 

.11    .1.5    .12    .18   .12 

!62 

'.SI 

.34   .14   .09 

.09 

.13 

2.30 

.19 

B 

"    6      " 

.12j  .16    .10    .14    .24 

.24 

.22 

.10   .40   .06 

.08 

.14 

2.00 

.17 

fi 

"     7      " 

.12!  -17    .08'  .13   .17 

.18 

!25i  .18    .28'  .08 

.07 

.15 

1.86 

.16 

7 

"    8      " 

.12]  .16    .08|  .13   .27   .14 

.17!  -11    -371  .15 

.23 

.15 

2.0s 

.17 

8 

"    9      " 

.12i  .1.5    .12    .12    .23    .06 

.27,  .30   .26!  .09 

.15 

.18 

2.06 

.17 

9 

'•  10      " 

.151  .13!  .lo!  .10  .18 

.06 

.21    .22     32 

.10 

.14 

.14 

1.85 

.16 

10 

"  11      " 

.13   .12 

.09 

.16   .26 

.09 

!24l  !l3i  -33 

.16 

.15 

.14 

1.98 

.16 

11 

"  Md't. 

.10   .10 

.15 

.14   .13 

.14 

.13    .21    .26 

.18 

.14 

.13 

1.81 

.15 

Table  XXVIII.  Total  hourly  precipitation  per  month  and  year.  The 
figures  expressing  hundredths  of  an  inch  of  rainfall  or  melted  snow, 
indicate  the  amount  of  precipitation  which  was  recorded  on  an  average  per 
month  and  year  during  each  hour  of  the  day.  For  example,  an  average  of 
10  hundredths  of  an  inch  of  rain  fell  from  noon  to  1  p.  m.  during  the  month 
of  March  from  1893  to  1902;  21  hundredths  in  July,  and  164  hundredths  on 
the  average  per  year  during  that  hour.  The  figures  are  based  upon  the  ten 
years'  record  of  a  self-recording  rain  gage. 

5  p.  m.,  then  decreasing  to  midnight  or  early  morning.  The  influ- 
ence of  the  thunderstorm  is  distinctly  seen  in  this  distribution  of 
rainfall.  The  intimate  connection  existing  between  the  intensity  of  rain- 
fall and  the  occurrence  of  thunderstorms  becomes  strikingly  apparent 
when  Fig.  44,  showing  the  hourly  distribution  of  rainfall  throughout  the 
year,  is  compared  with  Fig.   77,  showing  the  hourly   distribution   of 


MARYLAND   WEATHER   SERVICE 


167 


thunderstorms.     The  great  majority  of  these  local  storms  fall  within  the 
period  from  2  p.  m.  to  8  p.  m.  in  June,  July  and  August. 

The  early  morning  hours  of  June,  July  and  August  have  the  smallest 
amount  of  rainfall  (see  Table  XXXVIII),  while  the  hours  from  3  p.  m. 
to  7  p.  m.  of  the  same  months  show  the  heaviest  average  falls.  For  the 
month  of  September  the  heaviest  rains  are  apt  to  occur  somewhat  later  in 
the  day,  between  6  p.  m.  and  11  p.  m. 


i 

\  Jan. 


Fig.  45. — Average  Hourly  Amounts  of  Precipitation  in  January  and  July. 

The  amounts  are  expressed  in  hundredths  of  an  Inch.  In  obtaining  the  mean  amounts 
only  such  days  were  considered  upon  which  rain  fell  to  the  amount  of  .01  Inch  or  more. 

In  Fig.  45  the  average  monthly  amount  of  rainfall  for  each  liour  of  the 
day  is  shown  graphically  for  the  months  of  January  and  July.  The 
comparatively  uniform  distribution  throughout  the  day  in  January  is  in 
striking  contrast  with  the  small  rainfall  in  the  morning  hours,  and  the 
rapid  increase  in  the  early  afternoon  hours  in  July. 

Hourly  Eaixfall  Frequency. 

The  graphical  record  of  rainfall  during  a  period  of  ten  years  at  Balti- 
more, together  with  a  careful  record  of  direct  observations  of  beginnings 


168 


THE    CLIMATE    OF    BALTIMORE 


and  endings  of  rainfall  and  snowfall,  enable  us  to  make  a  careful  sinidy 
of  the  actual  and  relative  frequency  of  precipitation  at  different  hours 
of  the  day  and  night.  A  table  was  prepared  (see  Table  XXXIX)  show- 
ing the  number  of  times  precipitation  was  recorded  during  each  hour 
of  the  day  from  January,  1893,  to  the  close  of  December,  1902. 


TABLE  XXXIX.-AVERAGE  MONTHLY  FREQUENCY  OF   RAINFALL  FOR   EACH 

HOUR  OF  THE  DAY. 


a 

X5 

(i        (i 

>, 

§ 

>. 

bfi 

4i 

4J 

> 

d 

a 

a 

■^  00 

eg 

Hours. 

O 

,2     8* 

<a 

3 

2 

3 

o 

y 

0 

0 

0 

^  3 

3.1 

4.3 

4.0  3.8 

3.2 

•-> 

2.0 

1-5 

2.8 

1.7 

CO 
2.4 

0 
3.0 

3.8 

0 
3.0 

3.1 

<^^ 

Md't. 

to  1  A.  M 

37.1 

1 

"    3      "     

3.0 

3.7 

3.7  3.5 

3.7 

1.9 

3.4 

1.6 

1.6 

3.0 

3.8 

3.0 

2.9 

34.9 

2 

"    3      "     

3.0 

3.5 

3.6  3.6 

3.4 

1-  7 

3.1 

1.3 

1.3 

2.9 

3.6 

2.6 

''.7 

33.5 

3 

"    4      "     

3.3 

3.8 

3.9  3.4 

3.6 

1.8 

1.9 

1.3 

1.3 

2.8 

3.4 

2.9 

2.S 

33.4 

4 

'•    5      " 

3.2 

4.1 

3.8  3.S< 

4.1 

1.7 

l.H 

0.9 

1.4 

2.5 

3.4 

3.3 

2.8 

34.0 

6 

'•    6      "     

3.6 

4.3 

4.6  4.7 

4.3 

1.6 

1.9 

18 

1.8 

3.0 

3.7 

3.3 

3.2 

38.5 

6 

"     7      "     

3.7 

4.3 

5.2  4.6 

4.8 

2.0 

1.8 

1.8 

2  2 

3.2 

3.5 

3.3 

3.4 

40.4 

"     8      "     

4.4 

5.4 

5.7   5.5 

6.0 

2.9 

2.4 

2.  '^ 

3.5 

4.1 

3.7 

4.2 

4.0 

48.6 

8 

"    9      "     

.5.4 

5.6 

5.4  4.6 

6.6 

2.5 

2.9 

3.4 

3.9 

3.9 

4.1 

4.6 

4.2 

49.8 

9 

"10      "     

5.4 

5.3 

5.2  4.r, 

3.9 

2.2 

3.6 

1.7 

3.3 

3.5 

4.5 

4.4 

3.8 

45.5 

10 

"11      "     

4.8 

4.4 

5.1  4.5 

3.8 

2.3 

3.8 

1.9 

3.7 

3.5 

4.3 

4.2 

3.7 

44.3 

11 

"  Noon     

5.5 

4.6 

5.3  3.6 

3.9 

2.7 

3.3 

3.0 

3.6 

3.3 

4.1 

4.3 

3.S 

46.3 

Noon 

"    1  P.  M 

5.7 

4.6 

.i.2  4.4 

3.9  3.1 

3.3 

2.4 

2.7 

3.0 

4.5 

4.3 

3.9 

47.1 

1 

'*    3      "     

5.0 

4.2 

6.1  4.3 

4.0  3.1 

3.5 

2.6 

3.0 

2.8 

4.9 

4.2 

3.9 

46.8 

2 

"    3      "     

4.T 

3.9 

6.4  5.0 

3.8 

3.1 

3.5 

3  .0 

3.2 

3.0 

4.3 

4.1 

3.9 

46.4 

3 

"    4      "     

4.7 

4.3 

5.3   :>.2 

4.3 

3.5 

3.0 

3!i 

3.5 

3.8 

3.9 

4.3 

4.0 

47.8 

4 

"     5      "       

5.7 

4.7 

6.1  4.6 

5.0 

4.8 

4.4 

2.6 

4.1 

3.7 

4.0 

3.9 

4.5 

.53.6 

5 

"    6      "     

5.0 

4.3 

6.0  4.9 

4.7 

3.7 

4.3 

3.3 

4.0 

3.2 

4.6 

3.9 

4.2 

50.8 

6 

"     7      "     

5.3 

5.1 

5.0  4.8 

5.3 

4.1 

4.4 

3.9 

3.3 

3.3 

4.4 

4.7 

4.4 

53.4 

7 

"    8      "     ....".'..'.'.'....'...'.'...'.'. 

5.4 

5.1 

5.2  4.S 

5.4 

4.1 

5.0 

3.7 

3.3 

3.6 

3.9 

4.7 

4.4 

53.3 

8 

•'    9      "     

5.4 

4.7 

5.3  4.2 

5.6 

3.4 

4.8 

3^7 

3.4 

3.0 

4.3 

4.5 

4.3 

61.3 

9 

"10     "     

5.1 

4.8 

5.3  4.6 

4.5 

3.0 

3.3 

3.3 

3.8 

3.0 

4.3 

4.2 

4.0 

48.0 

10 

"11      "    

5.3 

6.2 

5.3  5.3 

3.9 

2.3 

3.5 

2.7 

3.0 

3.1 

4..") 

4.1 

4.0 

48.1 

11 

"  Md't 

4.9 

6.1 

4.9  4.8 

3.8 

2.3 

3.3 

2.0 

3.7 

3.0 

4.0 

3.5 

3.7 

44.3 

Table  XXXIX.  Average  monthly  and  annual  frequency  of  precipitation  for 
each  hour  of  the  day.  The  figures  indicate  the  average  number  of  times  rain 
or  snow  fell  per  month  and  year  during  each  hour  of  the  day.  The  results 
are  based  on  the  record  of  a  self-registering  rain  gage  for  ten  years,  from 
1893  to  1902. 


The  distribution  of  precipitation  throughout  the  day  is  quite  uniform. 
During  the  period  of  ten  years  there  were  but  few  months  in  which  rain 
or  snow  was  not  recorded  at  all  hours  of  the  day  at  least  once.  Precipi- 
tation has  most  frequently  occurred  between  4  p.  m.  and  5  p.  m.  in  the 
month  of  March,  namely,  61  times  in  10  years.  The  hour  of  least  fre- 
quency is  from  4  a.  m.  to  5  a.  m.  in  the  month  of  August,  with  a  total 
number  of  9  times  in  10  years.     On  the  average  there  is  a  fairly  well 


MARYLAND    WEATHEK   SERVICE 


169 


defined  diurnal  period  in  the  frequency  of  precipitation.  Beginning 
with  a  minimum  in  the  early  morning  hours  there  is  a  rise  in  the  average 
annual  frequency  to  a  maximum  at  about  5  p,  m.,  followed  by  a  return  to 
the  minimum  in  the  early  morning.  This  periodic  movement  is  most 
readily  seen  in  the  July  curve  (see  Fig.  46).  In  this  month  the  mini- 
mum frequency  (1.8)  occurs  at  about  6  a.  m.  and  the  maximum  (5.0) 
at  8  p.  m.  There  is  a  comparatively  rapid  increase  in  frequency  between 
7  a.  m.  and  9  a.  m.,  especially  in  the  winter  months.     The  most  uniform 


DT 

Mot 

. 

\J 

A 

A 

V           Jan 

\ 

/ 

\J 

V 

/' 

t 

/-^ 

^ 

/ 

_^ 

\J 

\         JULV 

\ 

■v 

/ 

■\y 

Fig.  46. — Average  Houi'ly  Frequency  of  Precipitation. 

The  curves  show  the  average  frequency,  for  each  hour  during  the  months  of  January 
and  July,  of  the  occurrence  of  precipitation  to  the  extent  of  .01  inch  or  more.  The 
values  are  based  on  the  ten  years'  record  of  a  tipplng-bucket  ralngage,  supplemented  by 
eye  observations. 


distribution  occurs  during  the  cold  months  when  the  rains  and  snows  are 
associated  with  the  regularly  recurring  cyclonic  disturbances.  In  the 
summer  months  the  diminished  effect  of  cyclonic  disturbances  is  particu- 
larly noticeable  in  the  reduced  frequency  of  early  morning  rains,  while 
the  increasing  afternoon  rains  are  due  to  the  summer  thunderstorms, 
which  reach  a  maximum  frequency  between  3  p.  m.  and  5  p.  m. 

The  month  of  March  exhibits  the  most  uniform  hourly  distribution  of 


170 


THE    CLIMATE    OF   BALTIMORE 


precipitation  frequency,  especially  from  about  7  a.  m.  until  midnight. 
Precipitation  is  more  frequent  at  all  hours  of  the  day  during  the  winter 
and  spring  months  than  during  the  summer  months.  In  Fig.  46  the 
January  curve  of  frequency  is  well  above  the  July  curve  throughout  the 
day.  This  seasonal  and  diurnal  distribution  of  precipitation  frequency 
is  graphically  shown  in  Fig.  47,  in  which  an  increase  in  the  intensity 
of  shading  represents  an  increase  in  the  frequency  of  rains  or  snows. 
The  figures  attached  to  the  heavy  black  lines  show  the  average  frequency 


Fig.  47. — Average  Hourly  Frequency  of  Precipitation. 

The  heavy  shades  show  the  time  of  most  frequent  occurrence  of  precipitation  during 
the  day  for  every  month  of  the  year.  The  small  figures  attached  to  the  irregular  curved 
lines  show  the  average  number  of  times  precipitation  was  recorded  per  month  at  the 
times  indicated. 

of  occurrence  per  month  for  each  hour  of  the  day  and  month  of  the  year. 
The  hourly  and  seasonal  distribution  is  shown  more  accurately  in  Table 
XXXIX,  in  which  the  average  frequency  is  recorded  to  tenths  for  all 
hours  and  months  of  the  entire  yesLT. 


Duration  of  Precipitation. 

Precipitation  is  not  as  continuous  as  it  is  generally  supposed  to  be.  If 
an  automatic  record  of  a  rainy  day  be  carefully  examined,  it  will  be  found 
to  be  made  up  of  numerous  showers,  some  of  them  perhaps  extending  over 
an  hour  or  two,  but  most  of  them  lasting  less  than  half  an  hour.     The 


MARYLAND   WEATHER   SERVICE 


171 


duration  and  continuity  depend  mostly  upon  the  position  of  the  locality 
with  reference  to  the  center  of  the  cj^clonic  disturbance  which  is  the  occa- 
sion of  the  precipitation.  As  the  character  of  the  storm  and  the  position 
of  its  path  depend  largely  upon  the  season  of  the  year,  the  duration  of 
the  accompanying  precipitation  is  found  to  vary  with  the  season. 

An  automatic  tipping-bucket  rain  gage  has  been  in  use  at  the  Balti- 
more office  of  the  Weather  Bureau  since  January,  1893.  The  ten  years' 
record  enables  us  to  obtain  accurate  values  for  the  duration  of  the  pre- 
cipitation. These  records  were  supplemented  by  direct  observations 
during  the  daytime.  In  a  later  chapter  of  this  report  some  attention 
will  be  devoted  to  an  analysis  of  the  character  of  the  rainfall  accompany- 
ing different  t3'pes  of  storms.  In  the  table  below  an  effort  has  been  made 
to  arrive  at  average  values  only  for  storms  of  all  kinds.  It  has  been 
found  desirable  to  compute  the  average  duration  for  three  different  con- 
ditions. The  first  line  of  figures  of  the  following  table  includes  "  traces  " 
of  rainfall,  i.  e.,  amounts  perceptible  but  too  small  to  measure  accurately 
by  the  ordinary  methods.  In  the  second  line,  only  rains  of  measurable 
amounts  have  been  considered,  or  precipitations  equalling  or  exceeding 
one-hundredth  of  an  inch  in  depth.  The  third  line  relates  only  to  pre- 
cipitations which  amounted  to  less  than  one-hundredth  of  an  inch,  or  to 
"  traces." 

AVERAGE   DURATION   OF  rRECIPITATION. 
(In  hours  and  minutes.) 


Class. 

Jan. 

Feb.  Mar. 

Apr. 

May 

June 

Jul  y|  Aug.  Sept.  Oct.  Nov. 

Dec. 

Year 

A.    Including 
"  traces." 

10.00 

1 
13.10  !l0.20 

11.00 

7.30 

4.30 

4.00    4.00    6.30    8.30  10.50 

10.00 

8.20 

B.    Excluding 
"  traces." 

11.10 

13.10     8.45  11.00 

7.00 

3.30 

4.00    3.30    5.15    8.05 

9.25 

9.20 

7.50 

C.    Traces  only. 

3.16 

3.00  1  3.20  1  4.05 

3.00 

2.10 

1.40  1  1.40  1  2.30    3.45 

1           1 

3.45 

2.66 

3.00 

Class  A  contains  what  may  be  regarded  as  the  most  trustworthy  fig- 
ures for  the  average  duration  of  precipitation,  as  these  averages  express 
the  entire  period  of  precipitation  in  connection  with  a  passing  storm.   The 


172 


THE    CLIMATE    OF    BALTIMORE 


rains  of  the  winter,  spring  and  autumn  months  show  the  influence  of  the 
more  frequent  cyclonic  storms,  while  the  rains  of  the  summer  months 


A  S  O  N  D         J 


/ 

\ 

/ 

/ 

/ 

^ 

1 

I. 

y 

/ 

/ 

\ 

/ 

, 

i 

f 

\ 

\ 

/ 

\ 

\ 

/ 

\ 

B 

/ 

^ 

/' 

\ 

V 

y 

\ 

J 

/ 

/^ 

\ 

^^ 

\ 

c 

y 

/ 

V 



y 

Fig.  48. — The  Average  Duration  of  Precipitation. 

The  upper  curve  (B)  shows  the  variation  in  the  average  duration  of  rain  and  snow 
storms  during  which  the  precipitation  amounted  to  .01  inch  or  more.  The  duration  is 
expressed  in  hours  and  tenths  of  an  hour ;  the  values  are  based  upon  a  ten  years'  record 
of  a  tipping-bucket  raingage  supplemented  by  eye  observations.  The  lower  curve  (C) 
shows  the  duration  of  light  sprinkling  rains,  or  light  flurries  of  snow,  with  amounts 
too  small  for  accurate  measurements. 

are  mostly  of  the  kind  accompanying  thunderstorms.     The  former  have 
a  duration  averaging  more  than  double  those  of  the  latter. 


r^IAETLAXD   WEATHER   SERVICE  173 

An  examination  of  the  figures  in  the  table  above  will  show  that  the 
average  duration  of  rainfall  or  snowfall  is  a  little  less  than  eight  hours, 
when  only  such  storms  are  considered  as  yield  an  appreciable  amount  of 
precipitation.  When  "  traces "  are  included,  the  average  duration  is 
somewhat  above  eight  hours.  The  summer  rains  are  less  than  half  the 
duration  of  those  of  the  winter,  spring  and  fall;  they  are  obviously  of 
the  thundershower  type,  while  the  rains  of  the  winter,  spring  and  fall 
occur  mostly  in  connection  with  the  cyclonic  depressions.  The  entire 
interval  between  the  beginning  and  ending  of  precipitation  of  each  storm 
was  considered  as  the  duration  of  rainfall  or  snowfall,  regardless  of  inter- 
ruptions in  the  continuity  of  the  fall. 

The  duration  of  the  actual  period  of  precipitation  is  something  quite 
different  from  the  duration  of  the  general  storm,  or  local  atmospheric 
disturbance  in  connection  with  which  the  precipitation  occurs.  Dur- 
ing the  passage  of  a  storm  over  any  given  locality,  there  are  likely  to  be 
many  beginnings  and  endings  of  precipitation  with  intervals  of  a  few 
minutes  or  a  few  hours  with  no  precipitation,  or  of  so  small  an  amount 
as  to  be  negligible.  There  is  a  certain  t}^e  of  storm  which  is  of  com- 
paratively frequent  occurrence  in  the  Middle  Atlantic  states — the 
"  northeaster ; "  this  storm  is  generally  accompanied  by  heavy  and  per- 
sistent rainfall.  But  even  in  this  class  the  intensity  of  rainfall  varies 
greatly  from  hour  to  hour  and  it  is  generally  made  up  of  severjil  showers 
with  intervals  of  several  hours  without  appreciable  rainfall.  A  list  of 
storms  accompanied  by  an  uninterrupted  rainfall  exceeding  24  hours  in 
the  city  of  Baltimore  during  the  past  ten  years  would  not  be  a  very  long 
one,  the  number  probably  not  exceeding  three  or  four  per  year. 

The  following  list  comprises  rains  of  exceptionally  long  duration, 
which  have  occurred  during  the  ten-year  period  ending  with  1903.  In 
these  storms  the  rainfall  was  practically  continuous,  although  in  most 
of  them  there  were  intervals  of  a  few  hours  during  which  only  light 
sprinkling,  or  misting,  rains  were  recorded. 


174 


THE    CLIMATE    OF   BALTIMORE 


RAIN  AND  SNOW  STORMS  OF  LONG  DURATION. 

Am't. 

1893,  Apr.  19-21    44  hotus  of  actual  precipitation.        1.14  In. 

1894,  Apr.  10-12   52       "                "  "  1.95  " 

Dec.  10-12   49       "                "  "  2.01  " 

1895,  Jan.  8-10     55       "                "  "  1.55  " 

Apr.  27-May   1    102       "                "  "  3.69  " 

Nov.   24-25     42       "                 "  "  0.13  " 

1897,  Dec.  3-5   44       "                "  "  1.18  " 

1898,  Feb.  18-21    47       "                "  "  1.18  " 

Dec.  3-4   44       "                "  "  1.27  " 

1899,  Feb.  11-13   54       "                "  "  1.50  " 

1900,  Feb.  16-17    41       "                "  "  0.40  " 

1902,  Feb.  20-22   51       "                "  "  2.53  " 

Nov.  24-26     44       "                 "  "  1.60  " 

1903,  Apr.  13-15   43       "                "  "  1.68  " 

The  long-continued  rains  generally  occur  in  connection  with  our 
''  northeasters/'  depressions  originating  over  the  Gulf  of  Mexico,  or  in 
the  West  Indies,  and  moving  northeastward  directl}^  over  ]\Iaryland,  or 
following  the  Atlantic  coast  line.  The  most  notable  case  in  the  list  of 
long-continued  rain  storms  is  that  of  the  spring  of  1895,  when  rain, 
though  sometimes  very  light,  fell  for  practically  103  consecutive  hours, 
beginning  at  8  a.  m.,  April  27,  and  ending  at  2  p.  m.,  May  1.  The  total 
precipitation  for  the  entire  period  (3.69  inches)  was  not  very  large, 
though  the  rate  of  fall  was  at  times  excessive.  There  was  no  well-defined 
storm  area  near  Baltimore  at  any  time  during  the  period.  The  bar- 
ometer was  high  over  the  New  England  states,  while  there  was  a  shallow 
and  ill-defined  depression  over  the  Gulf  of  Mexico  which  moved  slowly 
northward  and  eastward  some  distance  off  the  south  and  middle  Atlantic 
coast,  causing  a  steady  northeast  wind  at  Baltimore. 

Frequency  of  Precipitation  of  Stated  Amounts. 

A  table  of  monthly  and  annual  precipitation  as  usually  compiled  may 
lead  to  erroneous  inferences  as  to  its  agricultural  value.  The  beneficial 
effects  of  rainfall  depend  not  only  on  the  quantity,  but  often  to  an  equal 
extent  upon  the  time  of  occurrence  and  the  rate  of  precipitation.  A 
given  amount  falling  rapidly  is  of  less  value,  agriculturally,  than  an  equal 
or  even  less  amount  falling  more  slowly,  as  a  rule.  The  greater  portion 
of  an  excessive  rain  is  apt  to  find  its  way  to  the  streams  immediately, 
while  the  lighter  rains  will  soak  into  the  ground  to  be  utilized  later  in 


MARYLAND    WEATHER    SERVICE 


175 


the  processes  of  plant  life.     It  is  of  great  importance  to  know  the  exact 
seasonal  distribution  of  rainfall  in  order  to  determine  to  what  extent  it 


TABLE  XL.-NUMBER  OF 


DAYS   WITH   PRECIPITATION   OF 
OF  AN  INCH  OR  MORE. 


ONE    HUNDREDTH 


Year. 

a 

>-> 

<! 

o 

a 

s 

1-5 

3 
1-5 

bo 

D 
< 

t 

02 

o 

o 
I? 

c5 

c 

a 
< 

1871 

1873 

1873 

1874 

5 
8 
13 
12 
11 

11 
14 
13 
9 
15 

7 
8 
13 
8 
8 

14 
6 
9 
13 
13 

10 
14 
9 
8 
13 

13 
15 
11 
15 
IS 

6 
10 
14 
15 
11 

7 
14 

■J 

14 

6 
9 
14 
10 

8 

10 
9 

13 
8 
5 

8 
9 
8 
8 
11 

11 
9 

13 
8 

14 

16 
10 
11 
9 
14 

15 
13 
11 

7 
13 

13 

18 
21 
10 
17 

13 

16 

li 

4 
9 
14 
9 

8 

15 
8 
9 

10 
9 

10 
8 

11 
2 

9 

14 
13 
9 
5 
10 

10 
8 
9 
6 

11 

13 
14 
10 
8 
14 

10 
11 
11 
12 
16 

9 

8 
9 
16 
17 

104 
133 

147 
109 

18T6 

136 

1876 

1877 

1878 

1879 

1880 

144 
129 
133 
119 
154 

1881 

1883 

1883 

14 
17 
19 
15 
11 

16 
10 
11 
13 
13 

13 
13 
8 

13 
16 

4 
13 
12 
14 
11 

6 
10 
13 

11.1 
13.8 

n.5 

10 
11 
13 
19 
15 

8 
16 
13 
11 
13 

16 
14 
14 
15 
6 

13 
13 
6 
17 
13 

4 
8 
10 

9.7 

13.8 
13.5 

14 
14 
8 
19 
13 

13 
13 
14 
13 
16 

18 
13 
11 
8 
13 

14 
13 
19 
13 
13 

13 
11 
13 

13.5 
14.5 
13.3 

13 
11 
15 
11 
9 

7 
13 

9 
16 
13 

10 
13 
15 
13 
11 

8 
10 
13 

4 
10 

14 
9 
9 

11.1 
11.7 
10.4 

10 
21 
9 
13 

13 

17 
9 
17 
16 

18 

14 

15 
14 
18 
13 

9 
13 
13 
13 

7 

14 

8 
7 

9.3 
14.3 

13.8 

14 

8 
16 
10 

8 

13 
13 

7 
14 
6 

11 
13 
14 
11 

10 

13 
13 
6 

7 
11 

7 
11 
14 

9.8 
10.8 
10.8 

8 
10 
14 
13 

10 

13 
13 
8 
18 
10 

14 

9 
11 

9 

9 

15 
14 

8 

10 

9 

11 
15 
13 

11.8 
11.5 
10.8 

7 
13 

7 

10 
15 

8 

7 
13 

9 
15 

13 
10 

6 

8 

6 

8 

8 
11 
13 
10 

11 

7 
14 

13.9 
10.3 
9.3 

8 
13 
9 
2 

8 

8 
10 
16 
17 
12 

5 
9 
8 
10 
5 

10 

4 

4 
10 

7 

13 

13 

5 

9.5 

10.3 

7.3 

9 
13 
13 

9 
11 

7 

9 
15 
13 
16 

9 
3 
9 
11 
4 

6 
12 
13 

6 
10 

6 

7 
8 

9.0 
11.4 
8.3 

10 
10 
10 
8 
13 

10 

8 
10 
16 

9 

11 
11 
11 
9 
10 

11 

15 
14 

5 

8 

4 
9 

7 

10.3 
10.4 
10.6 

15 

8 
12 
13 

6 

16 

10 

7 

10 
14 

9 

8 
11 
11 
13 

0 

15 
9 

13 
15 
9 

11.9 
11.1 
9.4 

132 
149 
145 

18S4 

1885  

142 
130 

1886 

134 

1887 

129 

1888 

1889 

138 

164 

1890   

155 

1891 

143 

1892 

1893 

129 
133 

1894  

134 

1895 

114 

1896 

115 

1897 

1898 

1899 

1900 

141 
125 
118 
115 

112 

1902 

122 

121 

Averajfes,  1871-1880. 
1881-1890. 

129.7 
141.8 
126.6 

1871-l!l03. 

11.9 

11.3 

13.3 

11.0 

11.8 

10.5 

11.5 

11.1 

9.1 

9.3 

10.0 

10.9 

131.4 

Table  XL  shows  the  frequency  of  occurrence  of  an  appreciable  amount  of 
rain  or  snow  (.01  inch)  for  each  month  of  every  year  from  1871  to  1903, 
also  the  total  annual  frequency,  and  the  average  frequency  for  each  ten  year 
period  and  for  the  entire  period  of  33  years. 


may  be  counted  upon  at  the  critical  stages  of  plant  growth,  A  statement 
of  monthly  amounts  will  not  reveal  these  important  facts  with  sufficient 
accuracy. 


176 


THE    CLIMATE    OF    BALTIMORE 


Tables  have  been  prepared  to  show  the  total  monthly  and  annual  fre- 
quency of  appreciable  amounts  of  rainfall  in  each  year  from  1871  to 

TABLE  XLL— ANNUAL  NUMBER  OF   DATS  WITH   PRECIPITATION  OF    STATED 

AMOUNTS. 


Tear. 


1871 

1872 

1873 

1874 

1875 

1876 

1877 

1878 

1879 

1880 

1881 

1883 

1883 

1884 

1885 

1886 

1887  

1888 

1889 

1890 

1891 

1893 

1893 

1894 

1895 

1896 

1897 

1898 

1899 

1900 

1901 

1903 

1903 

Average 
Greatest 
Least 


Less  than 
.01  inch. 


24 
10 
13 
31 

24 
31 

37 

25 
30 

43 
43 
56 
43 
44 

56 
45 
54 
41 

46 

60 
50 
55 

39.0 

60 
10 


.01  to  .10 
inch. 


37 

66 
63 
50 

57 

63 
61 
63 
63 
73 

50 
69 
64 
61 

58 

57 
54 
63 
74 
67 

56 
50 
67 
63 
44 

53 
59 
45 
38 
50 

43 
46 
50 

56. 
74 
37 


.11  to  .25 
inch. 


23 

27 
31 
20 
28 

35 
18 
16 
24 


30 

28 
SO 
26 

25 
20 
33 
21 

35 

33 
16 
23 
21 
23 

21 

37 
34 
24 


15 

25.1 

38 
15 


.26  to  .JO 
inch. 


25 
16 
23 
20 
20 

22 

22 
19 

20 

23 

28 
22 
28 
33 
18 

23 
26 
17 
31 


18 
29 
17 
2fi 
23 

12 

20 

23 
26 
21 

21 
19 

23 

22  1 

33" 

12 


.51  to  1.00 
inch. 


11 

18 
23 
12 
21 

14 
18 

33 
13 
11 

15 

18 
16 
20 
16 

14 
16 
14 
21 
21 

24 
25 
17 
13 
14 

20 
24 
17 
23 
14 

16 
19 
21 

17.5 

25 

11 


Over  1.00 
inch. 


11 
16 
13 

10.6 
17 
5 


.01  inch 
or  more. 


104 
123 
147 
109 
136 

144 
129 
133 
119 
154 

132 
149 
145 
142 
130 

134 
129 
138 

164 
155 

143 

129 
133 
134 
114 

115 
141 
135 
118 
115 

113 
133 
121 

131.4 

164 

104 


Table  XLI  shows  the  number  of  days  for  each  year  from  1871  to  1903  upon 
which  rain  or  melted  snow  was  recorded  to  the  depth  indicated  by  the  figures 
at  the  top  of  each  column.  Amounts  less  than  .01  inch  were  not  recorded 
until  1882. 


1903  (Table  XL),  and  of  the  total  annual  frequency  of  falls  of  stated 
amounts  (Table  XLI).  These  tables,  together  with  those  referred  to 
later  showing  the  rainfall  frequency  and  amounts  for  each  day  of  the 


MARYLAND   WEATHER   SERVICE 


IV  4 


year  and  for  each  successive  pentad  and  decade,  give  most  of  the  facts 
necessary  for  a  detailed  study  of  the  influence  of  rainfall  upon  plant 
growth  in  the  vicinity  of  Baltimore.  Some  of  the  more  conspicuous  de- 
ductions from  these  tables  are  here  summarized.  Considering  only  days 
with  an  appreciable  amount  (0.01  inch  or  more),  there  are  on  the  average 
131  per  year.  The  limits  of  variability  are  164  and  104,  occurring  in 
1889  and  1871  respectively.  Such  days  are  the  least  frequent  in  Sep- 
tember and  October,  and  most  frequent  in  March.     With  an  average 


I l_  _  _  _l 


Fig.  49. — Variations    in   the   Annual    Frequency    of   Days    with    Appreciable 

Precipitation. 


maximum  frequency  of  13.3  in  March,  there  is  a  steady  decrease  to  9.1 
in  October,  followed  by  an  almost  uniform  increase  to  March.  With 
normal  conditions,  the  rainfall  is  ample  at  all  periods  of  the  year.  Dis- 
astrous droughts  are  of  rare  occurrence.  The  most  pronounced  dry 
periods  of  the  past  33  years  will  be  referred  to  in  subsequent  pages.  The 
variations  in  the  total  annual  frequency  of  rainy  days  from  year  to  year 
are  confined  within  quite  narrow  limits  (see  Fig.  49  ).  The  successive 
ten-year  averages  from  1871  to  1900  are  130,  142,  127,  respectively. 
Since  1895  the  annual  frequency  has  been  continuously  below  the  normal. 


178 


THE    CLIMATE    OF    BALTIMORE 


with  the  single  exception  of  1897;  from  1880  to  1891  it  was  almost  con- 
tinuously above. 

In  addition  to  the  days  with  an  appreciable  quantity  of  rainfall 
referred  to  in  the  above  paragraphs,  there  are  nearly  forty  per  year,  on 
the  average,  during  which  light  sprinkling  rains  or  mists  are  recorded. 
Their  distribution  throughout  the  year  follows  closely  that  of  the  days 
with  appreciable  rain.  While  the  individual  effect  of  these  light  rains 
is  small,  their  aggregate  annual  value  to  vegetation  cannot  be  neglected. 

The  most  frequent  quantity  of  rain  or  snow,  and  hence  the  most  prob- 
able quantity  to  be  expected  in  all  months  of  the  year,  is  some  amount 
from  0.01  inch  to  0.10  inch.  The  average  daily  rainfall  for  the  year  is 
0.32  inch,  neglecting  traces.  Hence,  as  already  pointed  out  in  the  dis- 
cussion of  the  temperature  observations,  the  average  value  is  not  the  most 
probable.  The  average  monthly  and  annual  frequency  of  stated  amounts, 
based  upon  a  record  of  33  years,  is  shown  in  the  following  table : 

FREQUENCY  OF  PRECIPITATION  OF  STATED  AMOUNTS. 
(Average  for  33  years.) 


Precipitation  in 

hundredths  of 

an  inch. 

Jan. 

Feb. 

Mar. 

April 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Year 

3.4 
5.4 
3.4 
2.1 
1.5 
0.5 
11.9 

2.5 
4.8 
2.0 
2.1 
1.6 
0.8 
11.3 

3.6 
5.0 
3.5 
1.9 
1.7 
0.9 
13.3 

2.9 

4.7 
2.7 
1.8 
1.3 
0.6 
11.0 

4.6 
4.8 
2.5 
2.2 
1.6 
0.8 
11.8 

3.0 
4.1 

1.7 
2.0 
1.7 
0.8 
10.5 

4.0 
5.0 
2.2 
1.5 
1.4 
1.4 
11.5 

3.6 

4.7 
1.9 
1.8 
1.5 
1.2 
11.1 

2.2 
3.8 
1.4 
1.5 
1.4 
1.2 
9.1 

2.8 
4.7 
1.3 
1.5 
1.2 
0.7 
9.3 

2.9 
4.1 
2.1 
1.8 
1.5 
0.7 
10.0 

2.8 
5.3 
1.8 
1.9 
1.0 
0.9 
10.9 

38.3 

0.01  to  0.10 

0.11  to    .25 

0.26to0.50 

0.51tol.00 

Over  1.00 

.01  and  over 

56.3 
25.4 
23.1 
17.4 
10.4 
131.4 

♦Average  for  20  years. 


Average  Daily  Eainfall. 
In  the  following  consideration  of  the  question  of  the  daily  amount  of 
rain  or  snow  which  falls  throughout  the  year  no  account  was  taken  of 
days  upon  which  less  than  0.01  inch  was  recorded.  The  period  covered 
in  determining  the  average  daily  rainfall  was  that  from  1871  to  1900,  or 
thirty  years.  The  total  amount  of  precipitation  for  each  day  was  then 
divided  by  the  number  of  days  upon  which  precipitation  occurred  to  the 


MARYLAND    WEATHER   SERVICE 


179 


amount  of  .01  inch  or  more,  and  tliis  value  regarded  as  the  average  daily 
rainfall  and  recorded  in  Table  XLTI. 


TABLE    XLIL— AVERAGE   AMOUNT    OF   PRECIPITATION    ON    DAYS    WITH    RAIN 

OR   SNOW. 
(.In  hundredths  of  an  inch.) 


Date. 

Jan. 

Feb. 

Mar. 

Apr. 

1 
May  June 

July 

Aug-. 

Sept. 

Oct. 

Nov. 

Dec. 

Ann'l 

1 

.24 
.16 
.15 
.32 
.25 

.35 
..30 
.34 
.33 
.29 

.33 
.21 
.39 
.24 
.39 

.14 

.27 
.13 
.35 
.21 

.33 
.25 
.13 
.43 
.30 

.24 
.27 
.11 
.18 
.20 

.26 

0.26 

.08 
.35 
.34 
.23 
.25 

.48 
.19 
.37 
.51 
.28 

.154 
.38 
.42 
.20 
.23 

.61 
.21 

.28 
.27 
.27 

.31 
.41 
.44 
.34 
.36 

.26 
.17 
.24 
.45 

0.33 

.29 
.25 
.35 
.46 
.23 

.14 
.34 
.31 
.48 
.33 

.46 
.37 
.14 
.19 
.25 

.39 
.13 
.35 
.45 
.37 

.31 
.35 
.16 
.15 

.37 

.34 
.61 
.31 
.33 
.40 

.32 

0.31 

.17 
02 

.51 
.23 
.35 

.33 
"2 

'.h 

.47 
.34 

.23 
.17 
.35 
.21 
.18 

.13 
.30 
.28 
.19 
.26 

.35 
.29 
.14 
.19 
.50 

.48 
.45 
.43 
.36 
.24 

0.29 

.34 
.18 
.28 
.31 
.33 

.25 
.49 
.61 
.22 
.16 

.17 
.44 
02 
!34 
.43 

.37 
.18 
.25 
.39 
.41 

.41 
.33 
.33 
.33 
.24 

.37 
.26 
.21 
.21 
.21 

.32 

0.30 

.35 
.20 
.30 
.40 
.41 

.24 
.33 
.33 
.49 
.21 

.44 
.25 
.25 
.47 
.44 

.86 
.30 
.72 
.23 
.57 

.29 
.49 
.44 
.39 
.33 

.40 
.54 
.70 
.17 
.18 

0.37 

.36 
.71 
.39 
.29 
.36 

.17 
.33 

'.I2 
.38 
.40 

.63 
.33 
.33 
.39 
.54 

.34 
.40 
.39 
.33 
.47 

.69 
.49 
.19 
.36 
.41 

.54 
.36 
.38 
.39 
.65 

.54 

0.41 

.35 
.33 

.60 
.11 

.38 

.55 
.78 
.46 
.11 

.56 

.30 
.47 
.50 
.13 
.18 

.43 
.50 
.31 
.11 
.36 

.53 
.35 
.37 
.33 
.56 

.15 
1.03 
.14 
.54 
.20 

.28 

0.38 

.16 

.28 
.48 
.17 
.57 

.84 
.36 

..17 
.33 
.34 

.60 
.39 
.26 
.45 
.68 

.80 
.74 
.13 
.93 
.23 

.16 
.30 
.57 
.34 
.47 

.28 
.35 
.20 
.28 
.12 

0.41 

.15 
.28 
.14 
.55 
.26 

.37 
.15 
35 
.10 
.57 

.06 
.33 
.38 
.17 
.21 

.17 
.08 
.04 
.49 
.60 

.30 
.23 
.86 
.27 
.46 

.37 

'.h 

.09 
.18 
.44 

.39 

0.30 

.53 
.33 
.31 
.39 
.48 

.31 
.36 
.43 
.29 
.24 

.14 
.19 
.30 
.35 
.37 

.07 
.21 
.35 
.39 
.17 

.20 
.16 
.50 
.64 
.23 

.37 
.35 
.38 
.29 
.11 

0.30 

.29 
.15 
.39 
.30 
.42 

.34 
.70 
.11 
.11 
.60 

.18 
.25 
.29 
.39 
.33 

.16 
.46 
.23 
.22 
.41 

.33 
.29 
.11 
.33 
.14 

.30 
.28 
.16 
.37 
.37 

.19 

0.29 

3 

S:;:;:::;.;;-;;::::: 

6 

H   

9 

10 

11 

1--' 

13          

U 

15 

16 

IT 

18 

19 

20 

21 

22 

23 

24 

25 

26 

2- 

28 

29 

31 

0.33 

Table  XLII.  In  determining  the  average  daily  amount  of  precipitation  in 
the  above  table,  the  total  precipitation  of  the  month  was  divided  by  the 
number  of  days  upon  which  rain  fell  to  the  depth  of  one  hundredth  of  an  inch, 
or  snow  to  the  depth  of  one  tenth  of  an  inch.  The  record  is  based  upon  daily 
observations  during  the  period  of  30  years  from  1871  to  1900. 

The  results  are  interesting,  among  other  reasons,  as  showing  the  great 
variability  in  the  amounts  for  adjacent  days,  even  in  values  representing 
an  average  for  thirty  years.  The  average  amount  for  any  particular  day 
is  at  any  time  likely  to  be  materially  altered  by  the  occurrence  of  a  single 


180 


THE    CLIMATE    OF    BALTIMORE 


heavy  rain.  The  heaviest  rains  occur  in  the  warm  months  of  June,  July, 
August  and  September,  while  the  smaller  amounts  are  confined,  in  the 
main,  to  the  colder  months.  The  influence  of  a  single  heavy  rainfall  on 
the  average  amount  for  thirty-one  years  is  clearl}^  shown  in  the  excep- 
tionally high  average  fall  for  the  27th  of  August.     The  average  for  all 

TABLE  XLIir.-AVERAGE  AMOUNT  OF  PRECIPITATION 
BY  PENTADS  AND  DECADES. 

Pentads. 
(Pentads  ending  on  stated  dates.) 


January. 

February. 

March. 

ApriL 

May. 

June. 

5th         .22 

4th         .25 

1st        .26 

5th         .28 

5th         .25 

4th         .29 

10          .32 

9          .34 

6          .29 

10          .34 

10          .34 

9        .m 

15          .31 

14          .36 

11          .36 

15          .21 

15          .33 

14          .32 

20          .22 

19          .32 

16          .27 

20          .23 

20          .32 

19          .41 

25          .30 

24          .35 

21          .30 

25         .29 

25        .;« 

24          .43 

30          .20 

26          .25 
31          .37 

30          .39 

30          .35 

29          .43 

July. 

August. 

September. 

October. 

November. 

December. 

4th        .39 

3rd        .49 

2nd        .29 

2nd        .21 

1st        .33 

1st        .26 

9          .31 

8          .46 

7          .48 

7         .29 

6          .32 

6         .32 

14          .39 

13          .39 

12          .43 

12          .28 

11          .37 

11          .34 

19         .38 

18          ..SI 

17          .57 

17          .20 

16          .36 

16          .28 

24          .44 

23         .30 

23         .35 

23          .31 

21          .26 

•  21          .33 

29          .42 

28          .44 

27         .40 

27          .44 

26          .36 

36         .21 
31          .27 

Decades. 


Jan. 

Feb. 

March 

April 

May 

June 

July 

'Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

1st  Decade  — 

.27 

.30 

.31 

.31 

.30 

.33 

.37 

.42 

.40 

.29       .34 

..34 

2nd       " 

.26 

.34 

.31 

.23 

.32 

.40 

.39 

.32 

.51 

.24       .25 

.29 

3rd 

.25 

.33 

.30 

.34 

.28 

.38 

.45 

.40 

.31 

.35       .30 

.25 

Table  XLIII.  The  average  amount  of  precipitation  recorded  during  each 
successive  five-day  period  and  ten-day  period  throughout  the  year  is  shown 
in  the  above  tables.  The  figures  are  based  upon  a  30-year  record,  and  repre- 
sent approximately  the  most  probable  amount  of  precipitation  to  be 
expected  within  the  same  pentads  and  decades  in  coming  years. 


August  days  is  0.38  inch,  while  for  the  27th  it  is  1.03  inch.  On  the  27th 
of  August,  1882,  the  amount  recorded  was  two  inches.  The  total  num- 
ber of  times  rain  occurred  on  the  27th  of  August  from  1871  to  1901  was 
but  5,  while  the  average  frequency  for  all  the  days  of  the  month  was  11.4. 
The  variable  character  of  the  rainfall  from  day  to  day  is  more  readily 
seen  when  represented  in  graphical  form  as  in  Plate  IX.     Some  of  the 


MARYLAND    WEATHER   SERVICE 


181 


smallest  daily  amounts  belong  to  the  month  of  October.  The  average 
fall  for  tlie  17th  of  this  month  is  but  .08  inch,  and  that  for  the  18th  but 
.04  inch.     The  general  average  for  the  entire  year  is  0.32  inch  per  day. 

TAI5LE  XLIV.-FKEQUENCY   OF  PRECIPITATION  ON  EACH  DAY    OF   THE    YEAR 

FROM  1871  to  1901. 

(Precipitation  of  .01  inch  or  more.) 


Date. 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

An'l 

1 

15 

11 

10 

11 

9 

13 

10 

16 

4 

6 

8 

9 

2 

l.S 

9 

14 

10 

9 

12 

11 

16 

4 

8 

9 

8 

3 

4 

16 

13 

9 

10 

12 

r, 

8 

6 

8 

13 

7 

4 

10 

15 

13 

11 

13 

13 

11 

13 

7 

7 

7 

14 

m 

16 

14 

14 

9 

14 

11 

16 

11 

5 

8 

6 

11 

« 

15 

13 

10 

10 

18 

11 

13 

10 

13 

10 

9 

9 

7 

10 

17 

11 

11 

13 

9 

10 

9 

11 

13 

11 

7 

8 

8 
15 

15 
9 

13 
15 

13 
14 

11 
11 

11 

9 

11 

6 

11 
9 

8 
9 

10 
5 

13 
16 

9 

9 

10... 

12 

13 

13 

15 

11 

13 

7 

10 

13 

4 

13 

8 

11 

18 

13 

13 

10 

13 

11 

13 

11 

13 

~ 

11 

10 

rj 

15 

14 

16 

14 

9 

14 

10 

14 

13 

13 

11 

11 

13 

13 

16 

.  15 

10 

11 

13 

13 

14 

13 

13 

8 

9 

14 

12 

14 

13 

8 

13 

7 

11 

13 

9 

13 

8 

13 

15 

11 
15 

9 

10 

13 
13 

14 
15 

14 
14 

7 
14 

11 
9 

15 
11 

13 
11 

6 

5 

9 
6 

13 
5 

16 

17 

15 

13 

13 

11 

7 

17 

8 

9 

13 

6 

13 

i'Z 

18 

13 

14 

8 

13 

14 

9 

10 

13 

9 

6 

13 

10 

19 

12 

13 

16 

11 

13 

9 

11 

9 

8 

6 

10 

9 

20 

13 

13 

16 

13 

11 

0 

13 

8 

9 

9 

11 

9 

21 

12 

13 

15 

7 

16 

11 

8 

11 

8 

13 

9 

13 

oo 

9 

11 

13 

9 

13 

11 

10 

15 

10 

13 

i-z 

14 

23 

9 

6 

13 

13 

14 

7 

K 

15 

11 

13 

17 

13 

24 

13 

s 

13 

13 

14 

(; 

11 

14 

7 

11 

13 

11 

25 

14 

19 

13 

13 

14 

k; 

13 

13 

9 

7 

8 

11 

26 

9 

14 

14 

13 

13 

11 

17 

9 

13 

13 

14 

20 

07 

10 
15 

11 

10 

13 
15 

13 

18 

13 
10 

10 

11 

19 

18 

5 
6 

6 

7 

13 
16 

7 
13 

13 
11 

2S 

29  

13 

14 

15 

9 

13 

14 

10 

9 

16 

13 

13 

30 

8 

8 

6 

11 

8 

10 

13 

14 

11 

8 

10 

31 

14 

13 

13 

13 

13 

13 

17 

13.1 
1« 

13.5 
19 

13.9 
IS 

11.6 
15 

13.2 

18 

10.8 

17 

11.4 
19 

11.4 
16 

9.4 
14 

9.5 
16 

10.5 
17 

10.9 
20 

11.3 

Greatest 

20 

4 

6 

8 

5 

7 

6 

ti 

5 

4 

4 

6 

6 

4 

Table  XLIV.  This  table  indicates  the  number  of  times  in  thirty-one 
years  that  rain  or  melted  snow  was  recorded  to  the  depth  of  one  hundredth 
of  an  inch  or  more  upon  each  day  of  the  year.  Thus  we  learn  that  rain  or 
snow  fell  but  4  times  in  31  years  on  the  3rd  of  January,  that  precipitation  was 
recorded  20  times  on  the  26th  of  December  during  the  same  period,  etc.  The 
days  of  most  frequent  and  least  frequent  preoipitation  are  also  shown  for 
each  month. 

July  and  September  have  the  highest  average,  with  0.41  inch,  and  Janu- 
arv,  with  0.2G  inch,  the  lowest. 


183 


THE    CLIMATE    OF    BALTIMORE 


As  the  daily  averages  are  so  variable  in  character,  they  have  been 
grouped  in  periods  of  five  and  ten  days,  and  average  values  determined 
for  each  pentad  and  decade  of  the  year  (see  Table  XLIII) .  By  this  pro- 
cess of  smoothing  out  accidental  irregularities,  we  obtain  values  which 
represent  more  nearly  the  most  probable  precipitation  to  be  expected 
upon  any  day  of  each  pentad  or  decade  (see  also  Plate  IX). 


J           F          M          * 

M            J 

, 

A          £ 

0 

N 

D         J 

/ 

\ 

r 

\ 

1 5 

V 

/ 

\ 

/ 

1 

\ 

V 

^ 

J 

1 0 

/ 

\ 

^ 

\ 

y^ 

-^ 

\ 

^ 

y 

5 

/ 

^ 

^ 

"N 

\ 

^ 

.^^ 

/ 

^ 

\ 

^-> 

/ 

/ 

A 

n 

Fig.  50. — Monthly  Frequency  of  Precipitation. 

The  maximum  monthly  frequency,  the  mean  frequency  and  the  least  monthly  frequency 
of  occurrence  of  days  with  an  appreciable  amount  of  precipitation  are  shown,  respec- 
tively, by  the  upper,  the  middle  and  the  lower  curves. 

Daily  Eaixfall  Frequency. 
A  matter  of  considerable  importance  to  agricultural  and  commercial 
interests  is  the  frequency  of  the  occurrence  of  rain  or  snow  in  appreciable 
quantities.  This  has  been  determined  with  great  accuracy  for  the 
vicinity  of  Baltimore,  especially  since  1871,  when  systematic  observations 
were  begun  bv  the  Weather  Bureau.     Here  asfain  as  in  the  determination 


MARYLAND    WEATHER   SERVICE  183 

of  the  average  daily  quantity  of  precipitation,  the  average  values  for 
adjacent  days  vary  remarkably.  Thus  the  average  frequency  for  Janu- 
ary 2  is  13,  while  that  of  January  3  is  but  4.  For  June  24  and  25,  the 
values  are  6  and  16  respectively.  The  day  upon  which  rain  or  snow  fell 
the  greatest  number  of  times  from  1871  to  1901  is  December  26,  namely, 
20  times  in  31  years.  The  lower  limit,  namely,  4  times,  belongs  to 
January  3,  September  1  and  2  and  October  10.  (See  Table  XLIV  and 
Plate  IX.) 

The  table  throws  an  interesting  side-light  on  the  mooted  question  of 
the  occurrence  of  "  equinoctial  storms."  Are  rains  any  more  frequent 
on  March  21  and  September  21  than  on  the  days  immediately  preceding 
and  following?  On  March  21  rain  fell  15  times  in  31  years;  on  March 
19  and  20,  16  times;  on  22  and  23,  12  and  13  times  respectively.  The 
average  for  all  days  in  March  is  13  times.  On  September  21  rain  fell 
8  times  in  31  years;  the  average  for  all  days  of  the  month  is  9.4  times. 
That  is,  an  appreciable  amount  of  rain  fell  on  only  26  per  cent  of  the 
September  equinoctial  days  of  the  31  years,  or  4  per  cent  below  the 
average  for  all  days  of  September.  These  figures  show  that  rain  is  not 
as  likely  to  occur  on  these  days  as  on  many  other  days  of  the  month. 

The  Probability  of  Eain. 

If  we  divide  the  actual  number  of  occurrences  of  rain  on  any  given 
day  by  the  number  expressing  the  total  number  of  years  under  considera- 
tion (in  this  case  31),  we  obtain  an  expression  which  in  a  rough  way 
represents  the  percentage  of  expectancy  of  rainfall,  or  the  rainfall  prob- 
ability, for  that  day.  Rainfall  in  the  middle  latitudes  is  too  erratic  in 
its  occurrence  to  place  much  reliance  upon  this  percentage  as  a  forecast 
for  any  particular  day ;  if,  however,  we  have  a  long  series  of  observations 
and  take  the  average  value  for  5  successive  days,  we  arrive  at  a  figure 
which  more  accurately  represents  the  most  probable  percentage  of  occur- 
rences of  rainfall  for  any  one  of  the  five  days.  This  has  been  done  in 
Table  XLV  and  the  results  graphically  represented  in  Plate  IX.  The 
curve  based  on  5-day  means  shows  some  periods  of  the  year  to  be  de- 
cidedly freer  from  rain  tlian  others,  although  there  is  a  fairly  uniform 


184 


THE    CLIMATE    OF    BALTIMORE 


distribution  of  precipitation  throughout  the  A'ear.  The  period  from  the 
middle  of  September  to  the  middle  of  October,  for  example,  has  shown 
in  ol  years  from  ISTl  to  1901,  an  average  rainfall  frequency  of  about 
28  per  cent.  The  month  of  March,  on  the  other  hand,  shows  a  record 
of  about  42  per  cent.  The  last  week  of  July  shows  a  probability  of  52 
per  cent,  the  highest  for  any  week  in  the  year.     The  average  daily  proba- 

TABLE  XLV.— RAINFALL  PROBABILITY  BY  PENTADS  AND  DECADES. 

(In  perceiitag-e  of  possible  frequency.) 

Pentads. 

(Pentads  ending  on  stated  dates.) 


January. 

February. 

March. 

ApriL 

May. 

June. 

5tli      37.4 

4tli 

41.8 

1st      43.7 

5th       32.0 

5th      35.4 

4th      40.3 

10       38.6 

9 

43.8 

6        40.6 

10       39.8 

10        41.0 

9       32.6 

15       40.6 

14 

44.6 

11        41.8 

15       36.0 

15       38.0 

14       36.6 

20       43.8 

19 

38.0 

16        45.2 

30       39.8 

20       37.8 

19       36.0 

25       86.8 

24 

32.2 

21       43.4 

25       33.6 

25       45.8 

24       26.0 

30       35.4 

26        40.8 
31       40.0 

30       45.8 

30       36.0 

29       39.3 

July. 

August. 

September. 

October. 

November. 

December. 

4tli      29.4 

3rd 

40.8 

2nd      29.6 

2nd      28.2 

1st       40.8 

1st     31.6 

9       36.0 

8 

30.6 

7       26.8 

7       29.0 

6       37.6 

6    31.4 

14       34.0 

13 

37.2 

12       35.6 

12       25.6 

11        40.6 

11    29.6 

19       31.4 

18 

38.6 

17       37.4 

17       25.8 

16       27.0 

16    33.2 

24       34.2 

23 

37.3 

23       38.4 

22       29.0 

21       35.4 

21    33.6 

29       52.2 

28 

29.8 

37       38.8 

27       36.0 

26        44.4 

26    44.3 
31    40.6 

Decades. 

Jan. 

38.0 

Feb. 

March 

April 

May 

June 

1 
July  lAug 

.   Sept.    Oct 

i 

1 
.  Nov. 

Dec. 

1st  Decade  — 

42.2 

40.2 

35.9 

38.2 

36.4 

32.3 

36.4 

26.0 

25.1 

33.2 

30.2 

2nd 

42.2 

41.3  i    43.7 

37.9 

37.9 

34.0 

34.4 

37.3 

35.5 

26. C 

31.8 

28.2 

3rd 

36.9 

39.1      41.1 

37.4 

41.0 

33.3 

42.5 

35.9 

29.8 

39.6 

36.5 

42.1 

Table  XLV.  The  figures  in  this  table  represent  approximately  the  proba- 
bility of  rain  or  snow  upon  any  one  of  each  of  the  stated  five-  and  ten-day 
periods  throughout  the  year,  expressed  In  terms  of  percentage  of  the  total 
number  of  similar  days  in  thirty-one  years,  or  of  the  possible  frequency.  For 
example,  the  probability  of  the  occurrence  of  rain  upon  the  15th  of  March 
(or  any  stated  day  from  the  11th  to  the  20th)  is  expressed  by  43.7%;  for  the 
15th  of  October,  by  26.0%.  If  rain  had  occurred  upon  every  15th  of  March 
and  15th  of  October  in  the  thirty-one  years  from  1871  to  1901  the  percentages 
of  probability  of  rain  upon  these  days  would  have  been  represented  by  100. 

bility  for  the  entire  year  is  36  per  cent;  the  highest  for  any  one  day  is 
64  per  cent,  namely,  for  December  26,  and  the  lowest  is  13  per  cent,  for 
January  3,  September  1  and  October  10.     The  probability  of  rain  is  less 


VOLUME  2,  PLATE  I 


b 


PRECinTATinS  I'DOBADILITY. 

A.  Precipitation  ti-onnoncy  for  each  day  of  the  year.  fEipressp,]  , 

B.  rrecipitati.m  freqiieiicy  for  each  successive  5-day  pcrloa  oi  i  ™  as  u^rcenlages  of  the  possible  freqiie 

C.  Average  amount  of  precipitation  for  each  day  of  tl 
U.  Average  amount  of  precipitation  for  each  successlv 


MARYLAXD    WEATHER   SERVICE 


185 


than  20  per  cent  on  but  few  clays  of  the  year,  and  does  not  often  exceed 
50  per  cent. 

The  probability  of  precipitation  at  Baltimore  on  the  following  days 
may  be  of  special  interest : 


Jan.    1,      48  per  cent. 

Feb.    22,   35 

Mch.  4,     42 

Mch.  21,  48 

Apr.   30,    16 

May   1.      29 


May  30,  35  per  cent. 

July  4,     35     "       " 

Sept.  1,     13     " 

Sept.  12,  42     " 

Dec.   25,   35     " 

Thanksgiving  Day  35  per  cent. 

S         O         N         D        J 


- 

/ 

y 

) 

f\ 

^ 

\ 

/ 

^ 

V. 

J 

/■ 

^ 

■\ 

^ 

\ 

- 

■- 

-- 

N 

^ 

y 

Fio.  51. — The  Monthly  Amount  of  Precipitation. 

The  upper  line  indicates  the  variations  in  the  maximum  monthly  rainfall  from 
month  to  month  ;  the  middle  line  shows  the  mean  monthly  rainfall  based  on  30  years  of 
observations ;  the  lower  line  shows  the  least  monthly  precipitation  recorded  during  each 
month  in  30  years. 

The  Monthly  Precipitation. 
The  usual  method  of  representing  the  precipitation  of  any  given 
locality  is  by  means  of  monthly  and  annual  amounts.  -  Owing  to  the 
great  variability  in  the  character  of  rainfall  and  snowfall,  a  great  many 
years  of  continuous  observations  made  under  practically  imchanged 
exposure  of  the  gauge  are  required.     The  vicinity  of  Baltimore  has  to  its 


186 


THE    CLIMATE    OF    BALTIMORE 


TABLE  XLVI.-TOTAL  MONTHLY  AND  ANNUAL  PRECIPITATION  FOR  87  YEARS. 


Year. 


1817.. 
1818. 
1819. . 
1820.. 


1821. 
1822. 
1823. 
1824. 

1825. 


1826. 
1827. 
1828. 
1829. 
1830. 

1831. 
1832. 
1833. 
1834. 
1835. 

1836. 
183". 
1838. 
18.39. 
1840. 

1841. 

1842. 
1843. 
1844. 
1845. 


1846... 
1847... 

1848... 
1849... 
1850... 

18.51... 

18,52. . . 
1853. . . 
1854... 
1855... 

18.56.., 
1857... 
1858... 
18.59... 
1860. . . 

1861... 
1862.., 
1863... 
1864.., 
1865.. 

1866... 
1867.. 
1868.., 
1869. . 
1870. . 


".25 
0.90 
0.70 
2.80 


3.30 
1.80 
5.60 
2.30 

0.62 

0.81 
i.03 
1.1,0 
5.50 

i.n 

U.38 
3.21, 
'2.81 
1.11 
1.96 

3.94 
3.10 
2.10 
3.. 50 
2.30 

6.10 
1.80 
1.60 
3.65 
3.40 

2.83 
3.92 

1.58 
1.03 
3.58 

1.70 
3.60 
1.30 
4.40 
2.50 

2.11 
3.50 
1.83 
7.06 
2.29 

3.70 
0.60 
3.33 
0.80 
1.19 

2.50 
0.90 
2.56 
3.4" 
2!o6 


2.80 
2.00 
1.90 
2.20 

5.40 
4.80 
0.70 
5.90 

2.87 

1.85 
3.13 
2.1,1 
4.40 
1.79 

2.1S 
2.33 
1.05 
1.93 

1.57 

3.41 
3.10 
2.90 
3.60 
3.30 

1.40 
3.35 
2.20 
1.45 
3.59 

1.83 
3.43 
0.94 
1.15 
2.43 

2.90 
3.60 
3.40 
4.90 
4.00 

0.50 
0.66 
1.61 

5.74 
2./,2 

1.79 

i.n 

A.  15 
0.14 
1.21 

4.90 
3.80 

2.21 
2.85 
1..50 


•S     •    - 


4.. 50 
3.00 
4.55 
3.30 

1.70 
1.30 
7.10 
4.30 
/,.53 

5.70 
1.13 
3.25 
9.10  I 

A. 02  j 

2.87  I 
1.80 
2.12  I 
1.92 
3.73 

1.64 
6.30 
4..=)0 
4.00 
3.70 

5.95 
2.40 
3.80 
3.00 
1.70 

3.54 
2.38 
3.70 
3.63 
5.90 

5.70 
3.90 
2.70 
4.70 


2.47 
2.30 
1.31 

6.36 
1.32 


S.tS 
5.79 
4.23 
3.62 

1.40 
3.90 

3.20 
3.64 
1.90 


1.50 
2.10 
3.70 
1.10 

2.10 
3.10 

1.80 
4.70 
0.67  I 

S.il 
2.U7  I 
3.37 
4.40 
1.57  I 

U.61 
2.61 
0.56 

2.1,7 
3.82 

4.23 
2.10 

2.80 
9.10 
4.30 

4..'i0 
4.. 30 
2.90 
1.60 
1.49 

2.38 
0.41 
0.81 
0.87 
3.85 

4.70 
7.80 
3.10 
7.20 
0.39 

1.48 
1.84 
4.33 
6.96 


5.13 

5.67 
6.25 
3.. 54 

2.83 

2.50 
1.50 
1.65 
0.50 
3.03 


2.60  9.00 

6.45  1.15 

4.10  1.30 

4.40  4.60 


5.10 
1..50 
2.10 
2.95 
1.59 


1.80 
1..50 
1.60 
5.03 
3.08 


0.21  3.86 

2.29  !  1.80 

3.18  2.28 

3.40  1  8.50 

3.i2  I  U.92 


1.01 
U.90 
5.33 
3.21 

1.83 

4.10 

4.20 
4.30 
4.50 
3.90  i 

2.75  ! 

4.00 

3.55 

4,00 

2.36 

5.77 
1.19 
2.96 
4.18 
3.08 

4.60 
1.70 
4  .30 
5.20 
0.91 

1.19 
3.23 

9.08 
2.74 
3.i8 

4.76 

2.12 
.',.10 
3.39 
4.92 

0.20 
0.63 
3.80 
1.38 
3.53 


1.37 
A.S6 
3.32 
5.15 

9.20 
4.90 
4.70 
4.10 
5.10 

4.35 
2.65 
0.90 
1.70 
2.93 

1.78 
3.36 
4.34 
\.IQ 
1.66 

1.20 
2.70 
0.60 
4.80 


0.92 
7.45 
4.90 
1.16 

2.1,1, 

2.38 

5.71 
3.53 
0.82 
3.93 

2.. 50 
3.00 
.1.03 
1.76 
3.37 


3.50  10.00 
4.10 
3.20 
2.20 


7.50 
4.35 
3.60 
3.37 

1.7i 


2.50 
/,.08 
4.64 
3.55 

S.6U 
2.2U 
3.62 
3.80 
5.78 

2.35 
4.30 
1.90 
5.60 
1.85 

1.35 

3.70 
5.40 
3.90 
1.26 


2.. 51 
4.42 

2.06 
3.10 

4.20 
5.70 
3.30 
2.60 
2.62 

1.82 
2.47 
3.23 

6.20 
0.77 

7.06 

2.11 
5.29 
0.41 
1.74 

2.15 
2.03 
5.05 
0.30 
0.35 


3.30 
2.00  3.20 
4.30  ,  3.00 
8.00     1.50 


0.30  lO.'iO 
0.80  I  2.25 
4.10  5.80 
4.50  2.94 
3.21  ,  2.U7 


2.1,0 
A.  95 
1.35 
3.98 
3.35 

k.6U 
/,.90 
2.9/, 
0.59 
1.81 

6.70 
5.10 
9.10 
2.20 
3.35 


1.92 
0.83 
i.28 
1.93 
2.16 

U.92 
1.38 
3.56 
3.33 
2.1,9 

3.15 

3.80 
4.50 
1.90 
3.80 


4.00  1  2.30 
4.40  !  1.00 
7.82  10.. 50 
0..31  ;  4.47 
1.51 


7.20 
3.97 
3.34 
2.55 
4.70 

3.30 
4.60 
4.70 
3.00 
2.50 

4.88 
4.43 
3.37 
3.76 

7.25 

3.31 

0.85 
1.30 
2.36 
1.90 

1.92 
3.. 52 
1.33 
0..50 
1.68 


3.88  j 
5.. 55  ' 
1.64 
1.90 
4.70 

0.50 
2.20 
2.40 
4.10 
2.30 

2.83 
1.40 
4.44 

7.05 
2.69 

1.80 
3.70 
0.91 
2.19 
1.81 

4.20 
1.00 
4.4S 
3.15 
1.76 


1.80 
3.10 
0.70 
7.80 

3.40 
3.. 50 
3.80 
1.77 

0.88 

U.5U 
U.60 
0.99 
1.72 
3.32 


2.60 
1.92 
2.51 
0.85 

4.00 
3.10  ' 
3.10 

1.60 
4.50 

3.80 
1.40 
1.97 
3.03 
3.73 

1.30 
3.38 

7.35 
6.37 
3.10 

3.20 
3.60 
4.40 
7.10 
3.70 

0.77 
3.89 
2.34 

2.38 
3./,9 

3.96 

3.69 
1.8/, 
1.33 
3.68 

1.55 
2.60 

0..50 
5.08 
3.00 


3.70 
2.00 
1.10 
2.70 

5.60 
5.10 
3.10 
2.27 
l.~23 

1.62 
3.95 
5.51 
3.32 
l,./,2 

1.65 
2.21 
1.89 
2.55 
2.69 

4.80 
3.40 
3.70 

2.80 
3.15 

3.30 
3.75 
4.25 
1.85 
1.23 

7.17 
2.. 54 
1.44 
l.Ofi 
4.30 

5.60 
7.90 
3.. 50 

7.:« 
1.20 

1.85 
1.87 
3.97 

3.20 
5.05 

5.48 
3.91 
2.30 
2.41 
2.. 50 

1.10 

2./,9 
3.. 50 
1.86 
0.28 


3.60 
2.60 
2.20 
1.90 

3.30 
1.20 
6.25 
2.25 

3.. JO 

1.16 

2.95 
0.25 
1.37 
/,.6S 

1.09 
U.69 
5.12 
2.11 

2./,2 

7.10 
3.60 
4.50 
8.80 
3.25 

5.10 
3.35 
3.90 
3.. 50 
3.43 

2.10 

2.38 
3.10 
4.44 
4.40 

1..50 
6.20 
2.. 30 
3.90 
3.60 

2.05 
6.33 
5.65 

3.15 
2.99 

1.36 

1.50 
/,.1S 
1.40 
5.90 

2.. 50 

\.'m 

3.90 
1.04 


48.55 
32.60 
28.75 
43.. 50 

50.20 
29.20 
44.. 55 
42.28 
36.35 

30.68 
33.69 
33.01 
53.26 
38.97 

37.40 
34.27 
41.38 
29.51 
34.10 

.54.63 
45.00 
47.10 
51.70 
37.. 50 

43.90 
35.10 

48.79 
32.46 
28.39 

46.66 

as. 01 

34.42 

30.63 
44.80 

38.10 
.51.. 50 
.36.00 
.59.20 
29.31 

22.87 
38.37 
46.06 
.55.64 
37.54 

43.. 55 
35.48 
43.97 
33.03 
33.32 

37.48 
33.90 
33.63 
27.34 
22.43 


:maryland  weather  service 


187 


TABLE    XLVI    CONT.— TOTAL    MONTHLY    AND    ANNUAL    PRECIPITATION    FOR 

87  YEARS. 


Year. 


1871. 
1872. 
1873. 
1874. 
1875. 

1876. 
1877. 

1878. 
187P. 

1880. 


1881. 
1882. 
1883. 
1884. 
188.5. 


1886.. 

1887.. 
1888.. 
18S9. . 
1890.. 


1891 

1893 

1893 

1894 

189.5 


189fi. 
1897. 
1898. 
1S99. 
1900. 


1901.. 
19ft?. . 
190:i. . 


1.55 
0.88 
4.27 
2.22 

sisi 

1.67 
3.80 
4.. 51 
2.. 59 
2.26 

4.84 
5.38 
3.16 
4.81 
3.07 

4.48 
2.. 57 
3.35 
4.22 
1.80 

4.89 
6.42 
1.78 
1.46 
4.67 

2.62 
2.05 
2.99 
3.. 50 
2.11 

2.45 
3.05 
3.81 


Averages. 


l821-]H.30... 
1831-1840... 
1841-l8ro... 
18.51-1,'W)... 
1861-1870... 


1.38 

a.fts 

1.46 

3.06 

4.74 

3.02 

3.18 

1.41 

2.91 

4. 72 

2.96 
1.87 
3.31 
1 .  55 
1.96 

5.68 
3.73 
4.69 
6.69 
4.40 

5.49 
4.69 
2.. '3 
2^53 
4.80 

5.. 52 
2.41 
4.43 
3.. '3 

0.8:5 

7.07 
5.13 
1.32 
5.47 
4.65 

0.65 
4.68 
5.43 


2.4B  3.33 

2.81  2.43 

2.75  2.18 

2.98  2.97 

2.00  2.67 


6.37 
3.60 
4.74 
1.65 

4.82 

7.59 
3.43 
3.68 
6.37 
1.60 

4.85 
3.49 
4.62 
5.71 
4.07 

7.94 
7.20 
1.38 
1.19 
2.94 

4.70 
2.40 
2.. 58 
4.93 
3.17 

3.58 
3.41 
4.40 


4.21 
3.16 
3..';0 
3.. 35 
3.40 


S         -. 


1.90 
3.06 
2.77 
6.65 
4.27 

1.90 
3.30 
4.19 
3.69 
3.07 

2.00 
2.14 
3.20 
2.65 
1.37 


2.03 
1.44 
6.31 
1.92 
1.49 

4.94 
2.2.3 
5!38 
2.74 
1.23 

2. .30 
3.42 
1.^2 
3!i7 
4.50 

7.07 


2.06 

2.44  2.. 57 

2.11  4.22 

8.70  6.82 

3.94  5.98 


2.48 
3.15 

3.. 52 
3.80 
7.42 

1.44 
3.19 
1.84 
1.89 
2.06 

5.53 
2.90 
3.29 


2.66 
3.66 
2.. 31 
4.11 
3.07 


3.11 

6.35 
3.78 
7.26 
3.04 

1.61 

6.88 
3.86 
3.29 
1.00 

3.67 
1.62 
3.33 


2.. 57 
3.73 
3.. 38 
3.64 
3.28 


2.82 
4.16 
0.94 
1.11 
2.85 

4.09 
3.53 

4.09 
3.92 
5.48 

7.81 
2.30 
8.08 
2.. 51 
6.31 

5.64 
4.44 
3.22 
6.17 
2.42 

5.45 

4.87 
2.26 
3.29 
2.83 

3.94 

2.57 
1.06 
2.16 
4.;34 

0.90 
4.30 
5.01 


3.44 
4.. 52 
2.51 
2.90 
2.96 


1817-1870  1  2.52     2.68  3..55  I  3.03  [  3.40    3.32 

1871-1880 2.63    2.53  3.64     3.48     2.97     3.30 

1881-189<I 3.77     4.55  4. ."4     3.06     4.13     4.89 

1891-1900 3.25    4.04  3.84     3.08     4.02     3.28 

1871-1903 i  3.£0  I  3.'.0  3.99    3.27  i  3.63  1  3.78 


is       z. 


6.15 

1..58 
2.90 
4. .30 
4.78 

5.64 
4.60 
4.66 
3.16 
6.47 

1.40 
4.ft2 
3.10 
9.43 


8.08 
8.32 
2.82 
li!03 
3.61 

7.79 

4.07 
1.8-< 
1.73 
3.40 

6.32 
6.93 
3..-1 

1.64 
1.51 

6.18 
2.45 
7.65 


3.92 
3..-1 
3.46 
3.29 
2.65 

3.34 

4.42 

5.45 

3.88 


3.41 

4.. 59 
9.49 
3.47 

8.67 


S.22 
5!06 
3.70 
4.83 
3.62 


1.76  10.  .52 

0.64  5.27 

4.82  0.82 

6.71  2.72 

4.44  1.78 


2.15 
5.10 
o .  7*^ 

L74 

7.78 

3.94 
4.15 
6.17 
1.40 
6.44 

4.24 
\.i3 
1.81 
1.41 
2.43 

1.93 
4.71 
6.09 

4.86 
2.91 

6.73 
4.31 

5.88 


2.89 
4.03 
4.00 
4.18 

1.87 


2.98 
9., 38 
3.49 
0.09 
1.30 

1.90 

2.80 
4.90 
4.59 
4.76 

5.46 
2.36 

1.80 
4.75 
6.01 

4.14 
2.17 
1.56 
7.09 
4.26 

2.10 
7.19 

l.no 


3.. 59 
3.18 
3.74 
2.99 
2.  JO 


3.11 

4.08 
6.21 
0.16 
1.44 

2.79  I 

5.22 

4!41 

0.75 

2.64 

4.06 
0.86 
2.83 
1.42 

6.51 

1.39 
1.06 
2.99 
4.12 
5.73 

2.76 
0.26 
3.44 

3.. HO 
2.20 

1.11 

3.67 
3.97 

2.09 
1.68 

l.f3 
6.85 
3.54 


2.65 
3.37 
3.43 
3.19 
2.52 


3.24 
3.17 

4.05 
2.48 
4.86 

2.74 
6.85 
3.55 
1.30 

2.86 

2.41 

0.65 
1.37 
3.09 
4.04 

4.09 
2.ft2 
3.04 
6.45 
0.74 

1.33 
3.85 
3.78 

1.98 
1.86 

3.34 
4.39 
4.34 
2.27 
1.81 


1.90 

o!97 
1.90 
3.14 

1.32 
2.23 

5.61 
5.33 
4.89 

5.90 
1.70 
2.9S 
3.91 
3.49 

3.12 
5.04 
3.26 
0.61 


3.24 

2 .  28 
2!29 
4.13 
3.t'4 

0.37 
3.40 
3.34 
1.40 
2.07 


3.26  7.07 
S.'O  5.66 
0.73    2.19 


3.61  2.68 

2.68  4.17 

2.99  3. .57 

4.14  3.77 

2.59  3.. 59 


3.59    3.17    3.06  i  3.14    3.30 


4.F0 
4.16 
3.23 


4.05 
3.63 
3.96 


3.08 
3.10 
2.50 


3..51  I  2.94 
2.79  3.17 
2.90     2. .54 


4.66    4.20  i  3.85    2.99    2.99    3.07 


33.74 
.34.76 
49.-37 
33.63 
45.26 

46.70 
43.14 
50.09 
36.01 
41.90 

49.13 
43.11 
40.. 53 

45.8'^ 
40.04 

.53.11 
43.-59 
43.53 
63.-35 
46.96 

.54.31 
45.05 
33.15 
38.. 33 
40.47 

3S.59 
47.49 
36.46 
40.. 59 
31.. 57 

43.04 
-50.13 
46.26 


38.01 
41.25 
37.83 
41.46 
:i-2.W 

38.13 

41-36 
47.32 
40.49 

43.:J4 


Table  XLVI  is  a  record  of  the  total  monthly  and  annual  precipitation  at 
Baltimore  from  1817  to  the  close  of  1903,  including  rain  and  melted  snow. 
From  1817  to  1824  the  record  is  that  of  Capt.  Lewis  Brantz;  from  1836  to 
1870,  that  of  the  U.  S.  Army  Medical  Department  at  Fort  McHenry;  from 
1871  to  1903  that  of  the  U.  S.  Weather  Bureau.  All  figures  in  italics  are 
interpolated  values  based  upon  the  record  of  the  Pennsylvania  Hospital, 
Philadelphia,  after  applying  the  proper  corrections  to  reduce  the  record  to 
the  Fort  McHenry  series.  No  attempt  has  been  matle  in  this  table  to  reduce 
the  entire  record  to  a  single  uniform  series. 


188 


THE    CLIMATE    OF    BALTIMORE 


credit  a  particularly  long  series  of  observations  made  under  careful  super- 
vision by  trained  observers.  From  1836  to  1870  the  record  contained  in 
Table  XL VI  is  that  of  the  U.  S.  Army  Medical  Department  kept  at  Fort 
McHenry.  This  is  followed  by  the  record  of  the  U.  S.  Weather  Bureau 
from  1871  to  1903.  The  observations  from  1817  to  1824  were  made  by 
Capt.  Lewis  Brantz,  in  what  was,  in  his  time,  West  Baltimore.     To 


I  — ■ — ' ' 

i  —  —  —  —  —  —  —  —  —  — I — ■ — 

2     

I     — ^    

0  ^_  1     M 


Fig.  52. — Mean   Monthly  Precipitation. 


complete  the  record  from  1817  to  1903,  it  was  found  necessary  to  inter- 
polate the  monthly  and  annual  amounts  for  the  years  1825  to  1835. 
This  was  done  by  computing  the  normal  precipitation  at  the  Pennsylvania 
Hospital  of  Philadelphia  and  applying  the  monthly  and  annual  de- 
partures from  this  normal  value  to  the  normal  amount  for  Fort  McHenr3\ 
The  records  from  1817  to  1870  are  fairly  comparable.  No  attempt  was 
made  to  reduce  this  series  to  that  of  the  U.  S.  Weather  Bureau  from  1871 
to  1903  in  Table  XLYI.     The  results  of  the  two  series  may  be  compared 


MARYLAND    WEATHER    SERVICE 


189 


with  entire  safety  by  employing  the  departures  from  their  respective 
normal  values.  This  has  been  done  in  Fig.  54,  in  which  the  departures 
are  graphically  shown  by  months  in  regular  chronological  order  for  the 
entire  period  from  1817  to  1903.  In  Plate  X  the  monthly,  seasonal  and 
annual  departures  are  shown  separately  for  the  entire  period.  The  Fort 
McHenry  series  may  be  reduced  to  the  Weather  Bureau  series  by  adding 
the  following  differences  to  the  monthly  and  annual  normals  of  the 
former.  These  differences  were  computed  from  an  overlapping  record 
of  twenty-one  years  from  1871  to  1891. 

Jan.      Feb.      Mar.     Apr.      May     June     July     Aajr.     Sept.     Oct.      Nov.     Dec.      Year 
0.88       0.55       0.78       0.5t       0.03       0.29       0.S4       0.05       0.0:5       0.47       0.48       0.49        5.4-3 

The  record  from  1871  to  1903  was  made  with  great  care  and  without 
any  interruption.  The  average  monthly  and  annual  values  computed 
from  this  series  may  be  accepted  with  entire  confidence  as  reliable  aver- 
ages for  Baltimore.  We  have  then  the  following  figures  to  express  the 
normal  precipitation : 

NORMAL  MONTHLY  rRECIPITATION. 
(In  inches  and  hundredths.) 

Jan.      Feb.      Mar.      Apr.      May     June     July     Aug'.     Sept.     Get.      Nov.     Dec.     Year 

3.20       3.70       3.99       3.27       3.63       3.78       4.66       4.20       3.85       2.99       2.99       3.07      4^3. .34 

While  the  above  figures  represent  the  best  average  values  available,  they 
do  not  represent  the  most  probable  values  to  be  expected  during  any 
future  month  or  year.  As  precipitation  is  the  most  variable  of  all  the  cli- 
matic factors,  a  long  .series  of  accurate  observations  is  necessary  to  estab- 
lish a  normal  average,  or  an  average  which  would  not  be  materially  altered 
by  succeeding  observations.  The  degree  of  variability  at  Baltimore  is 
indicated  by  the  figures  in  the  following  table  of  extremes  during  the 
period  of  87  years  from  1817  to  1903 : 

EXTREMES  OF  PRECIPITATION. 
(1817-1903.) 


.Ian.    Feb. 

Mar. 

Apr. 

May 

5.68 
1858 
3.20 
1866 
8.88 

June 

July 

Aug. 

6.41 
1817 
3.5(5 
1877 
9.97 

Sept   Oct.    Nov. 

Dec. 

Year 

.\l)(j\e 

normal 

1 

4.54    3.87 
18.59     1896 
2.32     3.05 
1873     1901 
6.86    6  43 

6.55 

1820 
2.80 
1894 
8.36 

6.04 
18:» 
2.67 
18.55 
8.71 

5.8S 
18:W 

2.SH 
1901 
8.76 

6.. 37 
18H9 
3.26 
18KI 
9.63 

7.53    4.86     4.76 
1831     18.33     1853 
3.76     2.H3     2.8»> 
1884     1874     1870 
11.29     7.69     7.62 

5.50 
3.05 

1.H2.H 

8  55 

21 .07 

1S64 

Helow 
Year.. 
KanKe 

normal 

15.70 

1870 

36.77 

190  THE    CLIMATE    OF    BALTIMORE 

The  extreme  monthly  ranges  are  observed,  from  the  above  table,  to  be 
more  than  double  the  average  monthly  amounts,  while  the  extreme  annual 
range  closely  approaches  the  mean  annual  precipitation.  Another  inter- 
esting fact  brought  out  by  the  above  table  is  the  ratio  existing  between 
excessive  and  deficient  monthly  precipitation.  In  every  instance,  except- 
ing the  month  of  February,  the  excessive  amounts  are  in  round  numbers 
about  double  the  deficiencies.  As,  in  the  long  run,  the  sum  of  the  ex- 
cessive amounts  must  equal  those  of  the  deficient  amounts  in  order  to 
produce  the  normal  value,  it  follows  that  a  precipitation  below  the  normal 
is  the  most  frequent,  and  hence  the  most  probable. 

It  is  the  excessive  rainfall  which  disturbs  average  values  to  the  greatest 
extent.  A  single  heavy  rainfall  will  occasionally  materially  change  the 
average  value  for  a  long  series  of  years.  A  case  in  point  may  be  cited. 
In  1897  the  average  precipitation  for  the  month  of  July  for  the  southern 
part  of  Anne  Arundel  County,  Maryland,  based  on  a  series  of  observa- 
tions covering  7  years  was  5.67  inches.  On  the  26th  of  July,  1897,  dur- 
ing a  local  thunderstorm  of  great  intensity,  a  rainfall  of  nearly  15  inches 
was  recorded.  This  single  fall  changed  the  average  July  rainfall  from 
5.67  inches  to  7.45  inches. 

The  Seasonal  and  Annual  Precipitation. 

The  great  variability  in  the  total  annual  precipitation  is  best  seen  by 
an  inspection  of  Fig.  53,  in  which  the  3'early  amounts  are  presented 
graphically  from  1817  to  1903  after  reducing  all  observations  to  the 
Weather  Bureau  series.  The  dotted  horizontal  line  represents  the 
average  height  for  the  entire  87  years.  The  fluctuations  from  year  to 
year  are  so  irregular  and  vary  so  greatly  in  amount  that  it  is  difficult  to 
detect  any  periodic  movement.  There  are  suggestions  here  and  there 
in  the  diagram  which  point  to  possible  periodic  swings.  There  are 
groups  of  years  during  which  the  precipitation  remained  constantly  above 
the  normal  value,  and  others  with  a  persistent  deficiency.  Take,  for 
instance,  the  period  from  1850  to  1861  (see  Fig.  53),  when  there  was  a 
decided  annual  excess  with  the  exception  of  two  or  three  years  in  the 
middle  of  the  period ;    this  was  followed  by  13  years  of  deficient  pre- 


^!    — =:^s— ^— 

I  ^^^^^^^^^^^^^  ^^^^^^^^^^^^^^ 

— I — ^^^^^^^^^^^^  ^^t^a^m^^mamm 

ZZ  Z^^  Z!ZZ  ZZZSZZ 

?  J — ^^= 

^— — — ^  ^^^"  ^^^"  ^^^" 


=  ^  ?; 


c:    ti  >  2 


.2    c  S^ 


*<  ^ 


i    i  •■«  •- 


fc    ?  5 


o  ?*  r 


192 


THE    CLIMATE    OF    BALTIMORE 


cipitation;  from  1873  to  1889  the  anuual  values  fluctuated  a  great  deal, 
but  on  the  whole  there  was  a  gradually  increasing  excess,  culminating  in 
1889  in  one  of  the  heaviest  annual  falls  recorded ;  in  the  following  years 
there  was  diminishing  rainfall  w  ith  an  average  Ijelow  the  normal  to  the 
present  time. 

TABLE  XLVII.-TOTAL  SEASONAL  PRECIPITATION  FOR  87  YEARS. 


u 

C      1 

h 

c 

t; 

a 

a 

bo 

c 

s 

a 

B 
O 

u 

bo 

C 

e 

^ 

3 

^ 

5 

be 

S 

S 

1 

p. 

a 

3 

3 

<     ; 

u 
a. 

3 

3 

© 
CO 

h 
M 

3 
IK 

<< 

! 

1817 

1817-8 
1818-9 
1819-20 

1820-1 
1821-3 
1822-3 
1823-4 
1824-5 

1825-6 
1826-7 

1837-8 
1828-9 
1829-30 

1830-1 
1831-2 
1832-3 
1833-4 
1834-5 


6.50  11.55 
5.20  11.35 
7.20  !  8.80 


132.58 
'  7.25 

7.80     .  ,. 
114.80  112.00 


8.80 
8.30 


1845-6      8.08  11.69  15.87  12.35 

1846-7    i  8.44  '■  3.98  i  8.84  111.47 

1847-8    '  4.90  '  6.47  11.90  10.43 

1848-9       5.27  8.68     6.11  9.23 

1849-50  10.45  12.83     9.46  12.10 


10.60     8.90  9.60  19.70 

9.90     4.90  6.65  ;  9.85 

7.50   11.00  9. .30  11.70 

14.45  11.95  12.90  6.98 

5.74     6.79  '  8.03  4.58 

6.03  ;  9.33  9.46  8.08 

6.33  I  5.89  9.31  9.3S 

6.83  '  9.80  8. .31  10.78 

10.15  16.90  17.13  6.97 

4.33    9.01  11.83  10.50 


jll.19 

6.66 
!  8.55 

I  8.83 
I  5.64 


8.49   11.36  10.05 

9.31     8.51  i  fi.19 

8.01  10.92  13.37 

7.60  I  7.71  I  8.39 

9.38  12.74  '  6.03 


1835-6  9.77  ,  9.97  18.35  11.95 

1836-7  !l3.30  13.60  14.30  10.30 

1837-8  7.60  11.60  15.70  10.30 

1838-9  11.60  17.60  11.90  '  6.30 

1839-40  13.40  10.90  9.30  '  9.45 

1840-1  10.75  13.20  9.70     8.40 

1841-2  110.25  10.70  10.75     5.15 

1843-3  7.15  ,10.25  14.12   16.72 

1843-4  I  9.00  8.60  5.91     9.35 

1844-5  9.49  5.55  6.96     6.46 


1850-1 
1851-3 
1852-3 
18.53.4 
1854.5 

1S55-6 
1856-7 
1857-8 
1858-9 
1859-60 

1860-1 
1361-3 
1863-3 

1863-4 
1864-5 

186.5-6 
1866-7 
1867-8 
1888-9 
1869-70 

1870-1 
1871-2 
18T2-3 
1873-4 
1874-5 


9.00  15.00  8.70     8.30 

7.70  13.40  13.00  13.70 

10.90   10.10  8.60   10.30 

11.60   17.10  10.40  18.50 

10.40     4.10  7.91     7.30 


6.31 
6.21 

9.77 
18.45 

7.86 


5.14  7.63  5.45 
7.37   14.35  6.16 

14.73  11.50  10.75 

15.96   11.13  13.61 

8.15  10.46  11.23 


8.48  13.71  12.75 

6.07  '  9.24  !  8.67 

8.98   16.14  10.12 

5.13  11.16  i  3.59 

3.80   10.36  7.57 


10.34 
11.33 
5.05 
5.93 
6.99 


13.30  4.16 
7.30  13.09 
7.24  7.71 
6.93  5.53 
7.40  i  7.45 


6. .57     6.85 

7.55  I  6.09 
10.01     8.48 

3.56  10.09 
5.40  I  5.04 


3.97  I  6.96  12.38  ,  8.57 
4.24  7.56  10.33  12.31 
11.23  11.60  13..S3  13.96 
6.37  9.98  8.88  7.47 
7.32  10.48  16.30     9.92 


1875-6 

1876-7 
1877-8 
1878-9 
1879-80 

1880-1 
1881-3 

1883-3 
1883-4 
1884-5 

1885-6 
1886-7 
1887-8 
1888-9 
1889-90 

1890-1 
1891-3 
1893-3 
1893-4 
1894-5 

189.5-6 
1896-7 
1897-8 
1898-9 
1899-00 

1900-1 

1901-2 

1902-3 

1817-70 

1871-03 


7.77  13.21   11.49 

6.99  !  9.13     8.77 

10.05  14.31  13.57 
9.75  8.08  13.79 
9.45     9.13  16.39 

15.41  11.89  11.. 36 

15.01  i  8.99  11.42 
9.55     8.10  13.90 

14.48  13.19  13.68 

11.38  I  7.47   16.76 

13.46  13.98  17.66 
10.38     8.50  16.91 

11.23  10.95  13.21 

10.01  21.23  18.60 

7.21  13.99  13.47 

13. OS  13.53   17.48 

13.07  16.70  10.77 

8.49  8.68  :  5.95 
7.38  12.25  6.43 
9.62  13.40    8.66 

13.53     7.75  13.19 

7.55  13.47  14.21 
7.71     8.28  10.66 

13.31  10.11  ;  8.66 

8.16  ■  6.33  '  8.76 

5.17  13.78  '13.81 
14.80  7.93  11.06 
14.90  11.03  18.54' 

8.50  10.01  10.35 
9.97  10.89  12.64  ; 


16.05 
17.34 

8.78 
4.77 
7.28 

9.45. 

10.89 
7.69 
4.60 

11.85 

7.38 

5.88 

10.93 

15.16 

11.23 

9.55 

6.47 

9.02 

10.53 

10.07 

8.59 
10.33 

9.87 
11.45 

7.75 

6.28 
17.76 
5.27 
9.37 


Table  XLVII.     For  the  character  of  the  record  in  the  above  table  see  foot- 
note to  table  XLVI. 


The  greatest  annual  precipitation  of  the  entire  period  of  87  years, 
making  due  allowance  for  the  permanent  differences  between  the  Fort  Mc- 
Henry  record  and  that  of  the  U.  S.  Weather  Bureau,  was  that  of  1854, 
with  a  fall  of  64.63.  The  mean  annual  amount  for  Baltimore  based  on 
the  Weather  Bureau  observations  for  33  years  is  43.34  inches.  The 
rainfall  and  snowfall  of  1889  followed  close  behind  that  of  1854  with  a 


MARYLAND   WEATHER   SERVICE 


193 


Jan.  Julv  Jan.  julv  Jan.  Julv  J 


Fio.  54a. — Departures  from  Moan  Monthly  Precipitation    (1817-1859). 


191 


THE    CLIMATE    OF    BALTIMORE 


Jan.      July      ■>*"  Jul/      J'"      J"i- 


JULv      Jan      Julv      Jan. 


4^44-186 

'+tt:           lU                 X-U               ^-  -1 tJ-H h-t-        --u 

' 

i  m          i              XX          =*b- 

— ,  .  . 

r   x         .,   3tlt-.^           iit           y'^C           i^ 

.-.-. . . 

V  \    ^    i^^ii    it   jk,     3tx  it  /X*  >    ^       J 

^\^r7V3rAy  ii    ^  a.>l/A  i-Hry  ,   ' V''  ' '   'yi    y"s/ 

i/.\i 

^¥  ^   ;^  v\.AW^/ny   .  .  ^^/^^^^^ 

-^'  7 

rr"T"-   -T    !  "  j-  I'n                "    '  :  ■ _...-    ^ .  _^-     ^ t--4-i' 

It 

M  '  '        -] — tHi                                 r _-      _i:xT 

I        1              '    ■      ^          '  I  I    '    1 

"  i^ 

A                               All                Ml 

i\  K                               i\                           Iv4   A                           V L 

/^, 

__    t-^. 

j^            V  A       *^     I  \        L    /    ^J     V     j^    J>      J'  ^  ^\>  n     \  /\    ;  \  V          L_                     /    "1/  * 

i? 

^f             \/\  /^ w    ^/^                   ^^  ^^                   V  \/           \    1^ t     / 

^ '                  V   Y/                                                                             '      •  '  V                *  ^     \     ' 

!      i      ,      1                       .      .                            1      ' 

1 

nil             11                 1    1 

T^                                    TV,, .  _  _.     _..    _. 

it  X                           ^iK      lit 

V                                 ^-^       -^    ni       t\t\            4\ 

iS           ^. .L^2_X/-S-/   r-    -jt-L ll iL 

:"iS?r 

"2: — ::x,'^  l7*s    ~^Lix3u  ^'  v^t  -      \  ^^^f'  ."  v^^^ 

^*'     \ 

i^\  ^t^t^"^    ^     5  2  jH/          4-  \v -_t-^ V  u^  *=/    y  - 

rr       .^ 

i                           ±                  X                    T 

X 

1  1                  -1                       !._..._ 

-)-         1875 

} 

X  ■                  _i_      XC             T~T^      """X    -XX 

i          1-     ilt  i           i  I         ^  •     A           A  — t-h 

+  ^ 

]   -,c    3:^^,111                  ^^  -       j^C^  Wa            a  At 

^^s-r      r 

J*        /n^?1/l        it                                        i4,>i^/V\          K.  y>.  f\       li  \ 

"  4'f\"7 

cc  s}^  /    [/    a\>  S/  L    '   yy       \i    \  A^' yz  " 

"  ^ '     / 

\r-   ^5^j^      v^   -^F-i^--'«^k             X^X     M  \  1     X-tt         A> 

^              1  '  '     -T-            "T~   ~^  V Ml        Y           ■ 

tx    it       ijit       X  r  ' '  X ;  "htifti  X 

t  1 

ir        1                 '8« 

0"                              -1881                   ^*   -   •   1882  j,^         +        ISSsI:                             1884 

■^5       1              1 

I          1                                                                 A  ,                                        L 

1 

1             1                                                                                          /)   .                                                        1 

i/^r~~^j'»k          i/\t  '       11          7"^^   I 

x    3k; 

t         ^-X.       XT    XTt?      S                     IIII         ^             it                     y          it     i    h     tl      - 

n  i;s-j' 

'r        /I      V   /I      A/    \l          j/    1       IALL  /I      J    J^      \i  /I       jTtJ 

°  3    l£ 

L^'        w.\  /v   ^::c^  X  J      ct    y'^7       "^.n    7 

S^    SL    " 

\^  i:          A^                     ^/-      i"-  '                  "  7 

T      1                                                               1                                                                                                                                                                   l\ 

xt             i                  X                  X      - 

i't'                                                                ik'''                 'll!                               l/1           i 

I'A'                     \/i                         lityil          1          'iji                                 il/ 

fl    /  Y      ^^       ^     ^                   '  '      /  V          A                   V^              i            \       ' 

>k '     /  A  i 

\/\v      w         1       iSi>^'i'*        '        a'a     /  ^^     ^  L  J           A    ^^ 

li                            u                   '       ^^/                1         1                        i         ^ 

V                                                                           1                             ]                                                                 '              ' 

;                 1                                                          1 

■18£ 

1  " 

1             yl      1  lI  1         1   i      [    ■    ""^     T               jc 

-jO"" 

A.^1-        A     -/V  i       jI  A  Al       1                       it                 /t 

-,^  A 

'Wu  J   .    I     /  VA      /    y  V7  \     .  .  i     1     ,  1          X   ^.-      X^\        .   -,' 

i^'lT  1 

*^x '/    ;v  y  V\  /  "  /  V  \     t-^  J    ,-k -it  ^t  4w  4-  -r->k-?- 

r*^  L' 

'   \/    yy  t\/  ^ :  X    7 \/r7 1^  T>  ^wC/    ^  ^t^* 

n — h  "V  " 

*^+\/  ^^+'* — ^^  M^  r  V/^     s^  \  -/    -'^u    w  ^ 

^      It             it   X               X^   It    '      ^^               "         v' 

X                            it                       X        - 

it       it           X         -    -                 X       - 

1     189 

XT" 

'    it          ^          ■   X                      -i^                                                                              Th       - 

31 

<,                            V                                                                             A          I                                                                                  it              r-                              It             - 

i\ 

/l                   J    >                   i>                                *          A        \           1-L                                        ii         ^L,       /5                           tl            - 

'   tJ 

f\          fi         /l«lAN\i\/^t            A      /\i^-t          ^^,       - 

\4^^l 

t\>*'   \  y  \AA  '^^   V  A/i  \  ^i  x^         vi*  yIi"* 

V/    ^« 

/   *"'       \_f    V\'\y    Y     '    V        V"   \>^r             ^tf       ^ 

3l 

'       V    T      ;                V     -^^      F-          -5'^ 

X 

X     X   it   X 

■■■- 

i 

1         1             1                      ---i-H-     -^-           1  1  1 

X           19( 

0           ~*~                  1901            X                  1902'                       ~^   1903"                                1904 

'X' 

X         it                               ,«!t                    X 

'     1 

"^        ±       lA                      J    \^\.                ^\                                     ^.^       - 

^S        3^       I                       ^    -U\/<       -,^\                                  JS 

A       i 

A              '  /\        i\      /    A        k    /       T  YlWl/    \  A                    J   l\  j^ 

rV  A 

A    -;  1    ii    lu/i^vAy                   \7\  ,?r  l^^'bt  S2^ 

rV/\j 

/  >'   t7     y      1-''         1  Ypr                            p^   v^^   af 

V  ' ' 

!                                                                        1                                                                                                           1 

1       1    1    1 

Fig.  54b. — Departures  from  Mean  Monthly  Precipitation  (1860-1904). 


1875  1880  1885  1390  1895  1900 

XRTCRKS    FROM    THE    NnRMAL   PRECIPITATION    (  1817-1904'). 


MARYLAND   WEATHER   SERVICE  195 

total  of  63.35  inches.  The  smallest  annual  precipitation  recorded  was 
27.86  inches,  reduced  value,  in  1870.  These  figures  show  a  total  range 
in  the  annual  precipitation  of  36.77  inches.  The  average  departure  of 
the  annual  precipitation  from  the  normal  quantity  is  6.68  inches,  a 
figure  which  attests  the  great  variability  of  this  climatic  element. 

If  we  compute  from  Table  XLVI  average  annual  values  for  each 
10-year  period  from  1820  to  1900,  we  find  a  fairly  close  agreement,  show- 
ing a  strong  tendency  to  return  to  a  certain  normal  value  in  spite  of 
great  fluctuations  in  individual  years.  The  most  conspicuous  departure 
is  the  great  deficiency  recorded  from  1861  to  1870. 

DEPARTURES   OF  TEN-YEAR  AVERAGES   FROM   THE   NORMAL   FOR   87   YEARS. 

Decades.  Departures. 

1821-1830 -0.11  inches. 

1831-1840 +3.13 

1841-1850 -0.30 

1851-1860 +  3.34 

1861-1870 -6.02 

1871-1880 -1.98 

1881-1890 +  3.88 

1891-1900 -2.85 

Monthly  and  Annual  Departures. 

The  chief  characteristics  of  the  monthly  and  seasonal  departures  for 
successive  years  are  clearly  shown  in  Fig.  54  and  Plate  X.  The  most  con- 
spicuous feature  of  these  curves  is  the  irregular,  short-period  fluctuation,? 
above  and  below  the  line  representing  the  average  amounts  for  a  long 
series  of  years.  The  extreme  irregularity  in  the  fluctuations  makes  it 
entirely  impracticable  to  employ  this  method  as  a  basis  for  making  long- 
range  forecasts.  When  the  precipitation  is  charted  by  months  in  regular 
chronological  sequence,  as  in  Fig.  54,  the  same  characteristic  fluctuations 
are  noted.  They  are  irregular  in  amount  and  period,  but  with  a  general 
tendency  to  return  to  the  normal  level,  and  with  occasional  evidence  of  a 
long  period  of  excessive  or  deficient  precipitation. 

An  inspection  of  Fig.  54  and  Plate  X  will  show  at  a  glance  that  the 

exact  average  precipitation  for  the  months,  seasons  or  the  year  is  an 

unusual  occurrence.     As  sliown  in  the  discussion  of  temperatures,  the 

arithmetical  mean  amount  is  not  the  most  probable  amount.     The  pre- 

14 


196 


THE    CLIMATE    OF    BALTIMORE 


cipitation  actually  recorded  is,  in  most  cases,  well  above  or  below  the 
normal  value.  The  following  figures  show  the  average  departure  above 
or  below  the  mean  value,  based  on  observations  for  87  years,  and  disre- 
garding the  sign  of  the  departures : 

AVERAGE  DEPARTURES  FROM  THJ:  MEAN   PRECIPITATION. 
(In  inches  and  hundredths.) 


Jan.  Feb.  Mar.  Apr.  May  June  July  Aug.  Sept.  Oct.   Nov.  Dec.  Year 


Plusorrainus 1.07     1.25     l.m     1.33     1.40     1.54     1.64     1.72     1.57     1.33    1.27     1.32 


The  most  frequent,  and  hence  the  most  probable  departure,  differs  from 
the  figures  above  representing  the  average  departure,  as  shown  by  the 
following  table : 

FREQUENCY  OF  PLUS  AND  MINUS  DEPARTURES  FROM  THE  NORMAL 

MONTHLY  PRECIPITATION. 

(In  percentage  of  total  frequency.) 


Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

46 
54 

8 

Dec. 

45 
54 

9 

Year 

Plus  departure  . . . 
Minus  departure.. 

40 
60 
20 

48 
63 
4 

46 
54 

8 

40 
60 
20 

46 
53 

47 

53 

5 

45 
54 
9 

46 
54 

8 

39 
61 
22 

45 
54 
9 

46 
54 

8 

In  all  months  of  the  year  the  departures  below  the  normal  are  in  excess 
of  those  above  the  normal ;  in  January,  April  and  September  as  much  as 
20  per  cent  of  the  total  number  of  months  in  87  years.  The  average 
monthly  difference  is  nearly  11  per  cent.  Hence  the  monthly  precipi- 
tation is  most  likely  to  be  below  the  normal  amount,  but  the  minus  de- 
partures are  likely  to  be  smaller  than  the  departures  above  the  norma). 

The  most  probable  departures  from  the  normal  monthly  amounts  are 
indicated  by  the  following  figures: 


FREQUENCY  OF  STATED  MONTHLY  DEPARTURES. 


Departures 
(In  inches). 

0—1 

1—2 

2—3 

3 — 4 

4—5 

5—6 

6—7 

7—8 


Plus. 


Minus. 


12.3     ' 

19.5      •• 
10.1      " 
1.4      " 
0.0      " 
0.0      " 
0.0     " 
0.0     " 

5.1     ' 

3.8     ' 

1.8     ' 

1.0     ' 

0.4     ' 

0.2     ' 

44.6  per  cent. 


55.1  per  cent. 


maryland  weather  service  197 

Excessive  Eains. 

The  heaviest  rainfall  recorded  in  Maryland  occurred  at  Jewell,  in  the 
southern  portion  of  Anne  Arundel  County,  on  July  27,  1897,  when  the 
local  voluntary  observer  measured  14.75  inches,  all  of  which  fell  in  the 
course  of  18  hours,  and  most  of  it  in  six  hours,  during  a  severe  thunder- 
storm. 

On  September  13,  1904,  a  storm  of  marked  intensity  developed  over 
the  Atlantic  Ocean  east  of  the  coast  of  the  South  Atlantic  states.  It 
increased  rapidly  in  intensity  during  the  14th  and  with  rapid  movement 
passed  northeastward  on  the  14th  and  15th  with  its  center  following  the 
coast  line.  The  precipitation  in  the  path  of  the  storm  was  of  unusual 
intensity.  The  center  of  the  cyclonic  system  passed  just  to  the  east  of 
Baltimore  during  the  night  of  September  14-15  accompanied  by  de- 
structive winds  and  torrential  rains.  The  rain  began  about  8  a.  m.  and 
continued  with  but  slight  interruptions  until  about  3  a.  m.  of  the  15tli. 
The  total  fall  during  these  19  hours  was  5.06  inches;  between  2  a.m. 
and  4  a.  m.  of  the  14th,  .02  inch  fell,  making  a  total  fall  in  24  hours  of 
5.08  inches.  This  is  the  greatest  fall  in  24  consecutive  hours  recorded 
at  Baltimore  since  the  establishment  of  the  local  office  of  the  TJ.  S. 
Weather  Bureau  in  1871.  The  greatest  rate  of  fall  during  this  storm 
occurred  between  8.25  p.  m.  and  9.10  p.  m.,  Avhen  l.iU  inches  were  re- 
corded in  45  minutes.  Some  of  the  more  interesting  records  of  tor- 
rential rains  or  "  cloud  bursts "  mentioned  by  Professor  Henry  in  his 
report  ^  on  the  rainfall  of  the  United  States  are  cited  here : 

"A  cloud  burst  passed  over  the  edge  of  the  little  town  of  Palmetto, 
Nevada,  in  August,  1890.  A  rain  gauge  that  was  not  exposed  to  the  full 
intensity  of  the  storm  caught  8.80  inches  of  water  in  an  hour.  At  Tri- 
delphia.  West  Virginia,  6.90  inches  fell  in  55  minutes  on  July  19,  1888. 
At  Campo,  California,  11.50  inches  fell  within  an  hour,  "  and  some  of 
the  fall  was  lost!" 

'Henry,  A.  J.  Rainfall  of  the  United  States.  Bull.  D..  U.  S.  Weather 
Bureau,  4to.  Wa.sh.,  1S97. 


198 


THE    CLIMATE    OF   BALTIMORE 


a»Ba 


Clr-li-(i-l 


i-rOl  i-l  r-t  Ci  i-IC^  i-l  gt 


qjaoK 


l^mv 


aot-«DiOO       -^»QiOCCi-l       i-O0CD»i^l™ 
OiOOeOi-ll—       O530000t-       ■ONtOl-'^ 

ei  N  -*  05  »i     CO  M  ?i  »i  P5     co  ci  oj » -* 


M  M  »-<  OS 

cj  o  i  i  CO 

Oj  «  r-l  CO  g-1 

33  «  3  a 


io»ccococo 


00 1-  to  £J  -* 
M  I-  U3  O  O 
CO  g^  f  1  -*  cc 


O  X  CO  05  t- 

-*  f— ( ct  *-<  -^ 


o  <M  >c  ei  1-1 

03  =  0© 

_fcj-5  <  X  -/. 
00  O  I-  O  "-I 

■>*  r-c  ■>*  cr.  » 


«3C0 

toiiffj 


■f  f  3      O 

31  »1  0>       to  "O 


CO  «  rl  51  ?5 


CI  X,  03        t~  OJ 


a^BQ 


■},niV 


*.niv 


CJ  ?■•  t-  i-t 

-#  ci  55  CO  -* 
00000 


C:00^-*        OlrtC".  ICCO 
0}  CO  1-1 1— I  ._  01  T?  1-1        irl_ 

Oj-i  o5  >H-H      -^00  r"  -^ 


•:^o:ct-;s 


f-l"  SSiCOl 


I— 1-#  c; 


C-. -iO 
OJi-lOl 

I    i   I 

•»«0O3 


■-l^Or-l-H        Qi-I  O  0--L 


OOiC  OS        OOr-l 

CCrHi-l        000 

...  .  o> 

OJi-i-H M-H 


i^CO  i^       o-^ 


ei  IM       rH  10       c-l 


C5      CO      »-^  tc 
1-1       1-1       r-Ci-l05 


c;coi--»~-a<     coeotocoi-i     Ni-fxicooi     omoocoo 

1— I  0>  07  ^H OJl-lClC> l-l  T-<f-<l-l  l-Hl-t  C<1 


•ON       -^ 
"AirtOctcO 


SKSJ2S?     SJ!S'~'SJS     1;°';?'*"^     <MO-*-*o     i-roo-^oo     i-it-ooi- 


CO ->!"■■•■ 

g>oi 


«  01  O       N 

CO-*'*     CO 


tcoscci-tco     oso-^tco      coi-it--^Tji     oi30f-i«co      lOoiotDo     C2»ao:5»o 
ooii-ioi-< 


000^^ 


oj  30  ii «  CO 

^Oi-<>^0 


Oi-li-IOi-l       t-l^i-lOO 


OMCO       =C^ 
oi  rH  O        oi  1-1 


a^BQ 


eoTjiojot-     co-*iMi-io 

C-if^i-IC'l        Ot       OiOICO 


ss 


1—1  CO  o  05      to  o  to  !0  CO 


J  01        OlOiOlOIOJ        OJ        1—1         CO        ^^1— IC 


co;5>-j     w^  <-i  r-{  r^ -j:,     co-*X) 


».niv 


e  ?*  01  ic  -* 

C0N-*r-(O 
i-i  M  CO  O  gt 


»-li*iOCOiO 

M*t-t-K5oo 
i-i  oi  oi  o  o 


01  :o  o  :c  Oi 


00 1—  X/  01  o 
OOOi-icO 


SS2^'*     ^'^^izn      tccsco      o^co 


O  01  33  X 1-1 


ooo:otD  X 

Or-ii-if-io 


O  l-  rl        Ml  t- 
...  .00 

1-1  01 1-1       CO  1-1 


ajBo: 


OJi-Irl 
to  to  CO  "OC 


1-100       CO 


1-H        kO        r-t  1-1  i-l 


tot-cot-o»     ^^rte^cj-* 


to  01 00 


xscocoxtc 


liOiv 


1— i-^OMOt.. 

O  tc  Ol  ■-(  00 


to        01  01 1-1 

I         III 
»o  CO  01  «0  10 


^OCsgsi— I        C5XCOXCO        -^COiOtCOi        O-i^iOtOCO        tOOlOOr^        OOtf^         tD»« 

0505'*co-H      "Ooicooo      i-xi-ixo     ooitoost-     ^-^=^^3      Socio      t-3» 


:i-lO^^^^       1—1  01  ^^00       OO^^Oi-t       i^^HO^^-.^ 


■^-f  1— (  Oi  to 

1-i  ^  -^  oi  CO 


to  OOKO       t-o» 
.     .  .00 

50     •■»^ 


ajBCr 


5i— (to  t-»0 


•-^  t:  S  S  **        I-  01  wOrl  1-t        ^  01  X  CS  -H 


O       t-1-1 
i-iiCNOl 

I    I    I    I 

005 -#  too 


_ej      01      01^  CO      ^  01  o; 


tj.tO« 
tOlO  t' 


l^rav 


nl2S52SS2     Si2SS2J^     S'^SSSJS     ;r55'^Si5'°     co-*oioob     uoot-toi^^i      xto  x 


■*ococo-a<     -^-^looco     Oi-;oi3DCO     e-it-o-d'os     -^xo^ti     o3x-*oo3     ojSx 

OOi-ioii-< 


1-I011-IOCO 


e^i  t-  o  -d<  OS 
oi  'H  oi  o  1-i 


■*xo- 

^Oi-<00 


aiBQ 


IM^Ol-r-^       COCtOllMOl       ^^Ol-^i-IOl       ^nOl       COOl 


^-       -H-HOi-liO 

Ol       01  O!  -^  CO  01  kO 

CO  g»  >c  —  o      too  0:0-*      XCOXOl-4" 


01 1—  X  to  o 
01  01 01 01 01 

01 01 01 01  ^^ 


^lOiv 


eSmSto°     SlSSSr:     SSSiS^     =£:S2i^S     <B«°tfo     2PS©»     o?-r^     not 
o<o>Hi-<oi 


1-1 01  e»  t- 1- 

coi-<oi^co 


XiO  t-.t-»o 


i-i£>050i-l 

o4aji-<^i-i 


iOXi-CT*il 

oioi^o^ 


Or-tCO  to  'f*! 

oi  oi  1-i  o  o 


oi-o> 


a^BQ 


XM0O5 
r-(Cjiej-* 

t-4<XClS>i 


ci  wo  t-. .-( 1-1 


OS        Ol-^ 

j-iol-5  61 


1-1         ice^ 


00  OS  tc  cox        OlOll-rtOI 


*itnv 


t- CO  to  to  00 
o  oi  o  o  o 


to  »o      XCO1— i:otD      to  I— I  to  cs  t 


01 01 01      1—1      1-1  01 01      01   _^  _  ^      ^^      ^  «>  .H 


OlC5-<iiC0  to 

i-<  o  oi  ^  oi 


■*X  to  X' 

oi  o  oi  o  ^ 


X  CO  01 01  CO 


ro-^^^  X 


1— li*:OCOC5       ototooo 

C0^H^^F.^O  Oli-HQ^^ 


rlKO        .-11-1 
II  II 

to  -#  CO  C-  "O 


o  t-.      ic  to  01 


Ol  X  i^  o  -^ 
1-100-^  ci 


O  — 1  .^  -j<  r 


8;Ba 


a^mv 


a;Ba 


i.niv 


KO  X 


>n  01     1-1      1-1 1-1 


J-H-<Xt- 


01     -<oi— <-^oi     1-1^0101 


Oi-J'OtOX      tooiot 


X  en  Ol  01 

t- 00 coo  to 


XiCCOtO-H        OlCOtOtOOS       MiiOtX       1-- 
01 1—1 01       1— I  ^^  1— I  ^H  <-<       01 01 01 


dCOi-ltOt- 

^  o  oi  o  o 


CO  X  Td  I-  WD 
oi  0 1-i  i-i  O 


QOL-7S<01        05XXOX        C^iOOXiil        .^COXCOCO 

~i-i/:^»-^,      csi-cooiio     osxSscsi?-     ^i-coaSS 


t- 1- 10 1-  CO 

OOOO-^ 


OlOi-lOl^^       O^^Ol^HO       0»Hi-100 


00-^ 


0:1-1  OS  01 

00  X  i  X  !— 

01  -H  — 


ox  xo 
CO  o;  1,- 1  -  o 


1-1  01  01       M< 


l-^tOCO"*       O^^OtOfc- 
SOlOtOtO      *#o»o»ox 


to  I—  01 1—1  to 

COCO&COlO 

0^-ii-<0 


10  X  o  10  ci) 

-^i-lOl 


010^ 

OlOlr-l 

1-1  US  00 


?       7 

X       i-IX-*t-X 


CO         r-1 
OJXCO 


oixxSS     S;2:S2:£;     eSSei:;     cotro     ooro 


otoo 


8^E(I 


;j,rav 


a^BQ 


^.tay 


a^Ba 


^^ray 


I-  CO  ■*  o  X 
oi-<o-4o 


XCOCS        »OQO 


1-l^HO        CO^H 


1-1       1-1  CO       OlNi-lol 


OlOSOtOl-       -*iCi-i01t-       OOt-OOSOO       1-IOI1— ICOOl 
1-1         Csl^H  eiOli— lOrOl  oil— 1-H  ^         l-f.^  <^> -- '  ■"  *^■» 


0000      coto 

Oli-lOSOlr-1 

ooo'iio 


01 1— 1  -H  Ol       CO  01  -H        01        01 1—1        01  -H       1-1 


101        -H 


i-llCOl 

1-1     01 


tOOS@C>l—        CO*Oii<tOOO        1— 11— -^OOCO        -i4«Ol-i*l-^00        MiOlCOCI^O        Xt  c^  r-~l  m  ^^ — ^^^«^  _t, — ^^  ^^ 

oosocoo     C000110C3S      «otoi;-.*io     iSieSSt^m      ^.AilAv^r^     SKSSS      !Si53^     r3S 


<oosoto>o      COOOIOCS 
i-^Qi-io-i     ci^oioo 


COQi-i^O 


1-^  01  to  t-  00 

iH-<i-ieio 


000 

^i-ioOC5 


OIUOOOOS       »Oi— i-^ji       1000 
-io^'-^0       -4-Hi-i        coS 


iHOlxO'Ci. 


1-101     rt 
-*  CO  01 1-1 01 

— I  ol  01  ^  1-1 


OS      1-1 
1-1     1-1 

00  OS  oco  cs 
^^^01 


i-H        1-p        Cjl        op        CI        1-10100 

oooMitot-     ri^oiw^t- 


„„»coco 

to  oil— 1-H  -H 

■c  1-1 00  01 01 


-!*>Oll-l 

cooco 


:^        CO— J»-li-H05        XC2-HOX        00  to  00  to        O  01  -H  to  '  -        00  O  -191  O  01        O^rv^ 

rt     otct-:o«5     tct-cocooo     tocoo^.*     Sosoioco     SSSSS!     S2S 


c>to  t-  :o«3 

OOi-i-i-4       i-ii-i-ioo 


t-  CO  CO  00 

jrtO-H-Hr 


tOCOOi-l-* 

oirt-H-ij-i 


•aosoioco 


-tO-H-* 


31^  t- 

001  i-i 


00  to 

i*OS 
•00 

CO  1-1 


CO         1-1     i-fosto 


10  OS  iO  to  t- 


O5t6ooo^-     oscocoooo      •«ji-*i-co'0     ^hou^osuo 

i-lr-IOl  1—11-1  0101i-li-li-(0101 

22E;S?S     S^J^Str     rSSS;^:*     2S^o»     M-ortoo 


H*l.^*<iOt-t-        l-'*00OSt-        -^05001.-1— 1 
00<-<00       0-H0>^0       i-<00-Hi-i 


OS  o  cs  01  -* 


i-*0S05C0 
l-ii-HQO-H 


■*-HO        01 
N  01 1-1 1- 1-1 

coo-aito—i 

OlNi-l       1-1 

ocsocso 
i-io^'oi-i 


i-INCO  01 

rt-HOi  CO 

1-101  01 

Iroos  in -J' 

cotoo  oscs 


-HdCOiinO        tOt-XCSO        1— lOlCO^iO         tCI^XOSO        -H01CO**1»0 


■t-Lr'^ir      trirtr'-oo      xxbooooo      xxooxS      ixcscedsSs      §^~c:o     S^3 
roxxoooo      ooxxoox      xxxxx      oo^xoox      SxxSS      xooxxS      §SS 


MARYLAND    WEATHER   SERVICE  199 

In  his  "  Climates  and  Weather  of  India/"  *  Mr.  H.  F.  Blanford  cites 
the  following,  among  others,  as  the  heaviest  rainfalls  on  record  in  that 
locality : 

TORRENTIAL   RAINFALLS   IN   INDIA. 

Place  Amount  in  ,,.. 

^'''^®-  34  hours.  ^'™®- 

Cherra  Poongee   f  Assam  i     40.8  June   14,  1876. 

Purneah   (Bengal)      35.0  Sept.  13,  1879. 

Nagina      32.4  Sept.   18,  1880. 

Danipur      30.4  Sept.   18,  1880. 

Rewah    (Central   India)     30.4  June    16,  1882. 

In  connection  with  the  above,  Mr.  Blanford  writes :  "  These  exces- 
sive falls  are  always  the  result  of  cyclonic  storms.  Not  such  as  are  of 
destructive  violence,  but  the  long-lived  cyclonic  storms  in  which  the 
barometer  is  not  greatly  depressed,  and  only  recognizable  in  their  true 
character  when  the  barometer  readings  and  the  winds  are  laid  down  on 

the  charts Another  noteworthy  point  is  that  they  have  frequently 

occurred  in  years  of  partial  drought Thus  in  1875,  although  in 

the  Punjab  it  was  one  of  the  wettest  years  on  record,  the  rainfall  was  very 
deficient  in  Bengal  and  all  over  the  southern  half  of  the  Peninsula.  .  .  . 
Scarcely  less  remarkable  is  the  occasional  heaviness  of  the  falls  even  at 
places  where  the  average  rainfall  is  not  by  any  means  excessive.  That 
upwards  of  40  inches  in  24  hours  should  have  been  recorded  at  Cherra 
Poongee  will  perhaps  hardly  be  surprising,  but  falls  nearly  as  great,  from 
30  to  35  inches,  in  the  same  interval  have  occurred  on  more  than  one 
occasion  on  the  plains  of  the  Ganges  Valley,  at  places  where  the  average 
for  the  whole  year  is  not  more  than  from  40  to  65  inches;  and  even  in 
the  extremely  arid  province  of  Sind  as  much  as  20  inches  fell  in  one  day 
in  1866,  at  Doprbaji,  where  the  annual  average  is  probably  less  than  six 
inches." 

Greatest  Kainfall  in  24  Hours. 

In  Table  XLVIII  the  greatest  precipitation  in  any  24  consecutive 
hours  of  each  month  and  year  from  1871  to  1903  is  recorded,  together 
with  the  amount  of  the  fall  and  the  date  of  occurrence.     The  greatest 

-  Blanford,  H.  F.  Tlip  Climates  and  Weather  of  India,  12mo,  London, 
1889,  pp.  77  et  seq. 


200  THE    CLIMATE    OF    BALTIMORE 

during  the  entire  period  for  each  month  is  also  graphically  shown  in 
Fig.  55.  Falls  equalling  or  exceeding  two  inches  in  24  hours  have 
occurred  in  all  months  of  the  year,  but  the  heaviest  have  been  recorded  in 
the  summer  and  early  fall  months.  The  greatest  fall  recorded  in  the 
table,  namely,  4.76  inches,  was  that  of  September  6,  1895.  During  the 
present  year  (1904),  however,  this  record  was  broken  by  the  excessive 
rainfall  in  connection  with  the  severe  coast  storm  of  September  14-15, 
when  5.08  inches  fell  in  17  hours. 

An  inspection  of  Table  XLVIII  shows  that  the  heaviest  precipitation  of 
the  month  may  be  very  small,  sometimes  falling  below  half  an  inch,  but 
such  instances  are  comparatively  infrequent,  especially  during  the  months 
of  active  plant  growth.  The  following  list  comprises  all  months  without 
a  fall  of  half  an  inch  or  more  in  24  hours  during  the  33  years  from  1871 
to  1903 : 

MONTHS  WITH   A   MAXIMUM   RAINFALL   OF  LESS  THAN   HALF  AN   INCH   IN 

24   HOURS. 

January    1S71,  1872,   1890. 

February     1805. 

March     None. 

April   1881,  1898. 

May  1872,  1896. 

June  1901. 

July  1894,  1900. 

August  1876,  1877,  1889,  1894. 

September  1878,  1884. 

October  1874,  1882,  1884,  1892. 

November   1882,  1890,  1903. 

December   1871,  1873,  1874,  1875,  187G,  1889,  1896. 

Fig.  55  shows  the  extent  to  wliich  the  heaviest  precipitation  in  24 
consecutive  hours  occurring  in  each  year  from  1871  to  1904  has  varied 
from  year  to  year.  The  amounts  range  from  a  minimum  of  1.47  inches 
in  August,  1898,  to  a  maximum  of  5.08  inches  in  September,  1904.  The 
tendency  to  a  periodic  fluctuation  embracing  a  group  of  years  is  more 
marked  in  this  diagram  than  in  those  representing  the  total  seasonal  or 
annual  fall.  Especially  interesting  and  instructive,  as  well  as  striking,  is 
the  gradual  and  steady  increase  in  the  intensity  of  maximum  rainfalls 
from  the  smallest  of  the  entire  period  of  34  years  in  1898  to  the  greatest 
of  the  period  in  1904.     It  would  seem  to  be  a  safe  inference  from  these 


MARYLAND    WEATHER    SERVICE 


201 


facts  that  we  have  arrived  at  a  maximum  for  this  particular  periodic 
swing,  and  that  during  the  following  two  or  three  years  there  will  be  a 
diminishing  intensity  of  precipitation  in  individual  storms.  This  view 
finds  additional  confirmation  in  the  grouping  of  excessive  rainfalls  of 
2.50  inches  and  over  as  sho\^ai  in  Fig.  56.  There  seems  to  be  no  fixed 
relation  between  the  annual  amount  of  rainfall  and  individual  rains  in 
any  given  locality.  Eegions  with  a  high  annual  or  seasonal  precipitation 
do  not  necessarily  have  excessive  rates  of  fall  for  shorter  periods.  While 
some  of  the  phenomenal  rains  have  occurred  in  the  tropics,  where  the 
seasonal  rainfall  is  generally  greatest,  there  are  many  instances  where 
record-breaking  downpours  have  occurred  in  comparatively  dry  regions. 


H 


Fig.  55. — The  Heaviest  Precipitation  in  any  24  Consecutive  Hours. 
(E.ypressed  in  inches  anfl  fractions  of  an  inch.) 

In  Table  XLI  will  be  found  a  record  of  the  annual  number  of  occasions 
on  which  the  rainfall  of  a  24-hour  period  equalled  certain  stated  amounts 
under  and  exceeding  one  inch.  The  precipitation  records  of  the  past  3-i 
years  have  been  further  examined  for  all  days  upon  which  the  rainfall 
exceeded  2.50  inches  (see  Table  XLIX).  Eainfalls  of  the  latter  amount 
may  be  considered  excessive  for  all  but  a  few  limited  regions.  They  are 
not  of  frequent  occurrence  in  the  vicinity  of  Baltimore;  since  1871 
there  have  been  but  42  all  told,  most  of  which  occurred  in  the  montiis 
from  June  to  September.  Their  total  monthly  frequency  in  34  years  is 
shown  by  the  following  figures : 


an. 

Kcl.. 

Mur. 

Apr. 

May 

•luni" 

July 

Aujf. 

Sei)t. 

Oct. 

\ov. 

Dec. 

^'(•al■ 

0 

o 

■> 

1 

1 

a 

9 

3 

9 

li 

1 

:! 

42 

202 


THE    CLIMATE    OF    BALTIMORE 


In  Fig.  56  their  frequency  and  intensity  are  also  graphically  shown  by 
months  and  years.  The  manner  in  which  these  excessive  rainfalls  are 
grouped  is  interesting.  There  are  apparently  three  groups  in  the  entire 
period  of  34  years,  of  which  the  j^ears  1876,  1887  and  1901  are  the 


JA'. 

18 
■    I       1 

7t                                 18 
1        1       1        1 

80                             18 
1       1        1        1 

85                              IJ 
1       1        1       1 

90                             IS 
1       1       1       1 

95                                19 
1        1        1        1 

00 

1        1         1       1 

Feb. 

1 

1 

MCH 

1 

1 

Apn 

1 

May 

1 

June 

1       1 

ll 

Juiv 

II 

1 

1       II 

1 

1 

AJG 

1 

1 

Sep- 

1 

1 

1 

ll 

1       1       1 

Oct. 

1       1 

II 

1 

Nov. 

1 

Dec. 

-I       1 

nil 

1       1        1       1 

nil 

1       1       1       1 

1        i        1        1 

ilM 

Fig.  56. — Rainfalls  Equalling  or  Exceeding  2.50  Inches  in  a  Day. 

The  frequency  and  seasonal  distribution  of  rainfalls  of  2.50  Inches  in  24  consecutive 
hours  are  shown  in  this  diagram.  The  total  amount  of  the  fall  is  roughly  indicated 
by  the  length  of  the  heavy  vertical  lines,  the  shortest  representing  2.50  inches. 


central  years.  These  years  coincide  with  considerable  exactness  with  the 
minimum  sunspot  period  of  approximately  eleven  years.  Further  atten- 
tion will  be  given  in  later  pages  to  the  relation  existing  between  rainfall 
and  this  well-known  period  of  solar  activity. 


MARYLAND   WEATHER   SERVICE 


203 


TABLE    XLIX. — DATES    UPON    WHICH   PRECIPITATION    EQUALLED    OR 
EXCEEDED  2.50  INCHES  IN  24  HOURS. 


.Januarj-. 

February. 

March. 

April. 

May. 

June. 

u 
o 

1       1 

s 

a 
C 

03 

i^  1    » 

f  1     © 

S        ai 

<     n 

Year 
Am't 
Date 

r- 

1880 
1883 

1885 
1SS6 
1900 

a 

< 

2.66 
2.66 
4.47 
3.18 
2.62 

1 

1886 
1896 

2.60 
3.48 

10-11 
5-  6 

1881 
1889 

3.. 51     8-9 
2.71     3-4 

1889 
.... 

3.58  25-26 

1886  2.99     7-8 

11 

26-27 

28 

22-33 

16-17 

July. 

August. 

September. 

October. 

November. 

December. 

1875 
1876 
1880 
1884 

1887 

1889 

1891 
1903 

2.70  1.5-16 
3.14        30 

3.71  20 
3.75        11 
2.77  20-21 

3.63     1-  2 
4.02  30-31 
2.. 59          8 
3.99  13-13 

1873 

1885 
1901 

4.36 
3.35 
3.28 

13-14 
2-  3 
6-  7 

1874 
1876 
1891 
1895 
1899 

1900 
1902 
1904 

3.15 
3.94 
4.00 
4.76 
2.  CO 

2.90 
3.61 
3.82 
5.08 

15-16 

16-17 

5-  6 

6 

19-20 

25-26 
1.5-16 
25-26 
14-15 

1873 

187.! 
1877 
187S 
1890 

1902 

3.42   19-20 
2.64  27-28 

2.74  4 

2.75  22-2-3 
3!04  ""  23 

2.79     4-  5 

1877 

.... 

2.85 

23-24 

18T8 
1888 
1901 

2.85 
2.. 56 

2.88 

.... 

10 
16-17 
28-29 

Table  XLIX  shows  all  periods  of  24  consecutive  hours  during  which  rain 
fell  to  the  depth  of  2.50  inches  or  over,  from  1871  to  1904.  The  day  and  year 
of  occurrence  are  likewise  shown,  and  the  total  amount  which  fell  within  the 
24  hour  period. 


1  I- 

1  1  1  1 

MM 

MM 
1 

MM 

J ...,.  ,   , 

I  1   1  r 

1 

1 

1 

II 

1 

1 

1 

II 

1  1 

1 

III 

III 

ml 

1     1 

1      1 

-I  1 

Mil 

MM 

MM 

MM 

<l<< 

Mil 

Fig.  57. — Rainfalls  Equalling  or  Exceeding  One  Inch  per  Hour. 

The  frequency  and  seasonal  distribution  of  rainfalls  equalling  or  exceeding  one  inch 
In  an  hour  are  indicated  In  the  above  diagram.  The  exact  amount  of  the  rainfall  is 
roughly  Indicated  by  the  length  of  the  short  and  heavy  vertical  lines.  The  double  lines 
indicate  the  oceurrenro  of  two  such  falls  on  the  same  month. 


204 


THE    CLIMATE    OF    BALTIMORE 


A  class  of  rainfalls  of  somewhat  greater  intensity  than  those  just 
referred  to  in  Table  XLIX  is  shown  in  Table  L,  which  contains  all 
occurring  from  1871  to  190-1  in  which  the  rate  of  fall  equals  or  exceeds 
one  inch  per  hour.  These  rains  have  occurred  almost  entirely  in  the 
warm  months  of  the  year.     None  are  credited  to  January,   February, 


TABLE  L.-DATES  UPON  WHICH  PRECIPITATION  EQLTALLED  OK 
EXCEEDED  ONE  INCH  IN  ONE  HOUR. 


May. 

June. 

July. 

August. 

-»:> 

® 

.^ 

.4^ 

« 

9 

^ 

+j 

» 

4) 

u 

-w 

e 

« 

e 

6 

eS 

a 

E 

a 

« 

OS 

a 

fc3 

£ 

s 

d 

^ 

< 

H 

o 

>* 

< 

H 

Q 

V* 

< 

H 

r- 

>• 

< 

H 

(5. 

1889 

1.20 

1-0 

20 

1881 

1.00 

0-45 

£0 

1877 

1.28 

0-55 

24 

1873 

1.30 

1-  0 

10 

1903     1.49 

0-37 

24 

1.16 

1-10 

£0 

1884 

1.40 

1-15 

31 

1875 

1.41 

1-15 

la 

1888 

1.10 

0-35 

33 

1895 

1.05 

1-  0 

•1 

1887 

1.74 

1-10 

22 

li-91 

1.15 

1-  0 

4 

1896 

l.EO 

1-  0 

21 

1888 

1.03 

1-  0 

8 

1893 

1.S3 

1-  0 

27 

1897 

1.36 

0-23 

17 

" 

1.13 

1-0 

5 

1901 

1.77 

0-25 

35 

1890 

1.96 

1-10 

21 

1903 

2.87 

0-33 

12 

1898 

1.43 

0-25 

1 

*' 

1.00 

0-24 

September 

October. 

:::: 

1899 
1901 

1.59 
1.00 

l.Ofi 

1-  U 
0-44 

0-35 

36 
6 

1896  '  1.00    0-40 

19 

1897 

1.38 

1-  0 

13 

13 

1899     1.00    0-53 

2.i 

1903 

1.40 

0-39 

5-6 

1900  i  1.62  1  1-  0 

1.1 

" 

1.23 

0-25 

37 

1904  1  1.48  i  0-40 

14 

'   

Table  L  is  a  list  of  all  occurrences  of  rainfall  equalling  or  exceeding  one 
inch  in  one  hour,  from  1871  to  1904.  The  year,  month  and  day  of  occurrence 
are  shown,  and  also  the  amount  recorded  and  tlie  duration  of  the  excessive 
rate  of  rainfall;  the  latter  in  the  column  marked  "Time,"  expressed  in  hours 
and   minutes. 


March,  April,  jSTovember  or  December.      The  distribution  through  the 
season  is  as  follows : 

May 


June 

July 

Aug. 

Stpt. 

Oct. 

Year 

5 

8 

13 

4 

1 

as 

The  monthly  distribution  here  indicated  associates  this  class  of  exces- 
sive rainfalls  at  once  with  the  thunderstorm.  Their  frequency  and 
intensity,  arranged  by  months  and  years,  are  also  graphically  shown  in 
Fig.  57.  The  grouping  referred  to  above  in  the  discussion  of  the  rain- 
falls of  2.50  inches  and  over  is  here  also  evident,  though  less  clearly. 


IMARYLAND    WEATHER    SERVICE 


205 


Excessive  Eates  of  Precipitatiox. 
The  rate  of  rainfall,  or  the  quantity  which  falls  per  hour,  or  part  of 
an  hour,  in  the  case  of  excessive  precipitation,  is  one  of  great  importance 
in  large  centers  of  population,  as  it  involves  the  engineering  problem  of 
providing  adequate  means  for  carrying  oi¥  the  surplus  water  without 
damage  to  proj^erty  or  interruption  to  traffic.  Especially  is  it  desirable 
in  this  connection  to  know  the  maximum  rate  of  fall.  Hence  particular 
pains  have  been  taken  to  tabulate  and  chart  excessive  rainfalls  under  a 
variety  of  conditions.  To  facilitate  the  study  of  such  practical  problems 
in  engineering.  Table  LI  has  been  prepared,  showing  all  the  necessary 

TABLE  LI.-EXCESSIVE  RATES  OF  RAINFALL  IN  CUMULATIVE 
FIVE-MINUTE  PERIODS. 


59  :>> 


Total 
duration. 


Begin-!  End- 
ning.  I    ing. 


Excessive 
rate. 


Excessive  periods  in  minutes. 


o    Begin- 
H     ning. 


?n^~  J  ®i  6  I  10  ,  15    20  I  25  I  30  j  35 


10 


45 


60 


1894.  6  6.6.5p 
S.lOp 
S.lOp 

"    20    8.20p 

8.20p 

1S94.23  ll.ir,!i 
1897.21  1.43p 
"  24  6.2Sp 
1898.  Ki  4.08p 
1899.1(5    «.50i) 


T.OOp 
8.45p 
lO.Oop 

9.22p 


7.30p  1  .25 

DN  1.47 

DX  L4- 

21st 

9.15a  1.53 

2lst 

9.15a  1.53  10.52P 

12.30p  .«)  l:.'.00n 

2.35p  .72    ].4!tp 

9.15p  .85    (i.KJp 

fi.09p  1.20    5.09p 

8.20p  .74    7.0.^1) 


1901 .  24  10. 15p  DX  .02  10.25p 
1902.25  n.3.-)p  6.20p  .51  ,'i.37p 
1903.,24    2.80a     4.20a  .l-OS^   2.53a 


7.04p    T   1.25     ...... 

8.57p   .20   .30  1.35  |.40  '  .. 
10.27p   .GO   .20   .30   .45   .55 


,15  i.30   .30 

,35  1.40  I  .. 


9.64p    T    .15 
11.06p  j.60  .15 


l.>.2'.p  .05  .20  .45  .60  .70 

2.00p  .01  .57  .67  .70  .71 

7.01p  .01  .11  .29  .42  .44 

ri.S'lp  .24  .29  .57  ..59  .59 

7.2<ip  T  .23  .42  .51  .53 


10.40p  .01  .11  .;50  .47  .48 
5..52p  T  .27  .to  .45  .47 
3.80a   .03  .21   .58  ,.60   .87 


40 


.75 


.45 


56 


.84 


Aver.. 
Great. 


Duration. 

h.  m. 
3-45 
12-65 


11.03 
1.63 

1       I 


Duration. 

li.  m. 
0-19 
0-37 


13 


28  .40 

.67  .67 


.93  1.33,1.441.611.64 


.66  .67 
,87  .93 


.83  .941.5i;i.e4 
1.321.441.511.64 


60 


80 


January. 

1896.26 

25th 
6.45p 

7.20a 

1.85 

1 

3.30a 

3.53a 

0.70 

.05 

.10 

.35 

.45 

March. 

1S99.  d  8.1.5p 

8.55p 

.34 

8.23p 

8.29p 

.01 

.01 

.26 

.29 

• 

M.\Y. 

206 


THE    CLIMATE    OF    BALTIMORE 


TABLE   LI   CONT.— EXCESSIVE   RATES   OF   RAINFALL   IN    CUMULATIVE 
FIVE-MINUTE  PERIODS. 


P 

Total 
duration. 

s 

S3 
1 

Excessive 
rate. 

Begin-   End- 
uing,     ing. 

■2® 

Excessive 

periods  in 

minutes. 

o 

Begin-    End- 
ning.       ing-. 

5 

10 

16 

20 

25 

30 

35 

40 

45    60    60 

60 

June. 


1894. 

1895. 
1896. 

1896. 
1897. 

iroo. 

1902. 

1902. 
1903. 

13 

24 

27 
8 
16 

21 

4 

25 
14 

7 

26 

6 

8 

4.28p 
4.10P 
3.00P 
4.04P 
4.17P 

4.47p 

4.33p 
3.50p 
4.10p 
2.13p 

6.27p 

8.20p 
3.03p 

5.00p 
6.30p 
7.35p 
6.20P 
7.03p 

5.25p 

5th 
8.18a 
4.07p 
8.10p 
4.55p 

26th 
4.00a 
7th 
7.10a 
3.45p 

0.47 

.70 

.65 

.95 

1.21 

.45 

.82 
.30 
.62 
.88 

1.13 

1.05 
.73 

4.40p 
4.57p 
3.02p 
4.07p 
4.20p 

4.56p 

4.48p 
3.5.3P 
4.20p 
2.13p 

11.25P 

1.46a 
3.18p 

4.54p 
5.08P 
3.08p 
4.36p 
4.40p 

5.14p 

5.05p 
4.03p 
4.40p 
2.43p 

11.50p 

2.04a 
3.35p 

T 

T 
T 
T 

T 

T 

.05 
T 
T 
0 

.33 

.25 
T 

.05 
.33 

.33 
.25 
.25 
.30 
.35 

.25 

.16 
.10 
.04 
.13 

.15 

.10 
.32 

.21 
.35 

.42 
.35 
.30 
.50 
.55 

.36 

.40 
.30 
.10 
.23 

.32 

.24 
.64 

.35 
.55 

.47 
.40 

.65 
.65 

.40 

.49 

.36 
.35 

.30 

.33 

.70 

.46 
.70 

.80 
.70 

.46 

.50 

.52 
.60 

.45 

.40 
.73 

.67 
.80 

.85 

.58 
.75 

.63 

.68 

.90 

.87 
.56 

.78 
i.90 

1 



— 



■■ 

_' 



•; 

V 
V 

V 

V 
V 

V 
V 

ver.. 
reat. 

Duration. 

h.  m. 
4-23 
15-45 

.75 
1.21 

Duration. 

b.  m. 
0-18 
0-30 

July. 


1894. 
1896. 


1897. 


1898.19 

"    120 

"     !28 

1901.25 

1902.  20 


3.35p 

13.30p 

4.30p 

10.23p 
12.25P 

7.25p 

9.15p 
7.45p 
2.10p 
12.25P 

1.45p 
8.25p 
2.17p 
6.00p 
1.27p 


5.00p    .44    4.41p 
1.30p  1.0512.21P 
4.5.5p     .66    4.33p 
5th 

5.10a     .33  10.28p 
12.55p     .50  12.26P 


7.43p 

.49 

22nd 

1.50a 

2.05 

8.38p 

.56 

3.25p 

.30 

5.40p 

1.76 

9.19p 
8. lip 
3.17p 
3.28p 


1903.12  IS.Olp 
"  I  ",  5.40p 
"     30    4.10p 


4.50p 
12.46p 
4.52p 

10.32p 
12.43p 


r.28p  ;   7.36p 


4.25p  1.01  2.0"p 

I)N  .51  8.30p 

3.15p  .88i   2.23p 

7.00p  1.95  6.00p 

1.65p  .56  1.29p 

l.lOp  2.87  12.04p 

7.60p  1.02  6.20p 

6.10p  1.04  4.33p 


Aver..  , 
Great. 


Duration. 

h.  m. 
1-66 
6-47 


1.00 

p.87 


10.39p 
8.24p 
2.23p 
3.48p 

2.35p 
8.44i> 
2.43p 
6.3.5P 
1.42p 

13.37p 
6.40p 
4.65P 


Duration. 

h.  m. 
0-21 
1-20 


.27 
.20 


.40 


T    .25  i.45 


T  .25  1.35 

T  .30   .45 

,03  .25   .26 

,38  .26   .6b 


.13  .36 

.15  .34 

.3?  .66 

.31  .80 

.31  .50 


.45 


.50 


.99   .99 


991.04 


1.05 


.60 


1.17 


.65 


.81^  .91 


.65  .70 


.95 


.58 

.50   .. 
.79j  .86; 
1.181.541.771.851.911.94 

.55   .56   ..':....      .. 


75  .86 


1.20 


.33   .98  1.722.23i2.52!2.693.87 
.34    ..54     .61    .71    .72    ..      .. 
.03   .18     .64   .9311.00,  ..      .. 


.03   .25  :.50 
1.38  1.37   .98 


.74   .88 


1.32 


1.732.232.622.692.87 


1.63 


1.23   .94   .951.201. 
1.9411.041  .9511.201. 


MARYLAND   WEATHER    SERVICE 


2or 


TABLE   LI   CONT. 


-EXCESSIVE   RATES   OP  RAINFALL   IN   CUMULATIVE 
FIVE-MINUTE  PERIODS. 


Total 
duration. 

6 

0 

Excessive 
rate. 

0 

1- 

Excessive  periods  in  minutes. 

Begin- 
ning. 

End- 
ing. 

Begin-    End- 
uing,      ing. 

5 

10 

15 

20 

35 

30 

35 

40 

45 

50 

60 

80 

August. 


1895 

31  I  4.18P 

5.00p 

.78 

0 

.78 

1 

1897 

9-10  11.30P 

5.10a 

l.«0 

1.30a 

2.20a 

.40 

.03 

.07 

.13 

.19 

.25 

.43 

.52 

.62 

.77 

.83 

" 

23    11.0:3a 

11.5-<a 

.74 

11.33a 

11.55a 

.02 

.14 

.24 

.48 

.71 

" 

25-61  9.50p 

l.lua 

.83 

11.26p 

11.37p 

.24 

.23 

.36 

.37 

V 

1898 

1  1   3.10p 

4.00p 

1.44 

3. lop 

3.40p 

T 

.46 

.78 

1.16 

1..36 

1.43 

V 

1893 

4-5  11  40p 

DX 

1.47 

11.50p 

13.25a 

.03 

.05 

.29 

.48 

.78 

.94 

1.03 

1.09 

.13 

1.17 

1899 

13 

2.1.5p 

2.40p 

.53 

2.18p 

3.33p 

T 

.23 

.43 

.52 

.53 

}■' 

" 

21 

7.29p 

9.10p 

.78 

8.10p 

8.24p 

.35 

.21 

.32 

.38 

.39 

y 

" 

26 

8.10p 

DN 

1.63 

8.15p 

O.OOp 

T 

.04 

.35 

.49 

.60 

.83 

1.00 

1.39 

.51 

l.SH 

) 

1900 

16 

2.20a 

3.00a 

.36 

3.21a 

2.30a 

T 

.18 

.32 

.34 

1 

1900 

21 

DN 

7.30p 

1.48 

5.47a 

fi.20a 

.25 

.10 

.37 

.58 

.72 

.85 

.91 

.93 

.94 

.97 

1.00 

1.06 

1901 

6       DN 

r2.25p 

ZA>b 

11.15a 

11.45a 

1.08 

.05 

.09 

..33 

..5"? 

.77 

.9'7 

" 

12 

10.37a 

2.05P 

1.43 

11.4.5a 

12.20p 

.09 

.OP. 

.10 

..31 

.43 

.78 

1.00 

1.10 

V 

" 

27 

6.35p 

7.30p 

.81 

6.40p 

6.55p 

.02 

.:50 

.62 

.76 

.78 

V 

1902 

3 

9.30p 

9.50p 

.tit) 

9.33p 

9.43p 

T 

.m 

.64 

.66 

]' 

1902 

5-6  11.40p 

D.V 

1.43 

11.46p 

16.25a 

.01 

.10 

.30 

.49 

.56 

.64 

.89 

1.34 

1.40 

Y 

" 

11     :5.11p 

4.  CM) 

.55    3.46p 

3.58p 

.14 

.17 

.35 

.41 

y' 

" 

27      4.29P 

5.30p 

1.2: 

4.3:p 

5.02p 

T 

.10 

.47 

.89 

1.12 

1  .-i'i 

1.24 

y 

1903 

26      9.45a 

10.3op 

.52 

9.53p 

lO.llp 

T 

.11 

.37 

.49 

.63 

*^ 

U\nA5p 

8.05a 

1.53 

4.30a 

4.41a 

.98 

.03 

.37 

.49 

.60 

Duration. 

Duration. 

h.  m. 

h.  m. 

Aver... 

2-48 

1.09 

0-24 

.18 

.15 

.35 

..51 

.65 

.85 

1.2;^ 

1.35 

1.13 

1.05 

.91 

1.06 

Great.. 

12-50 

2.05 

0-50 

1.08 

.46 

.78 

1.16 

1.36 

1.43 

1.24 

1.39 

1.51 

1.56 

1.00 

1.06 

September.* 


1894 

8 

1.27p 

1896 

3 

2.46p 

'* 

19 

4.66p 

1897 

16-7 

10.38p 

1898 

26 

6.20p 

1899 

2 

5.30a 

25 

5.20p 

1900 

15 

1.20p 

1.55p 
3.431 


1.51p 
3.57p 


.i.i.ip    ..in     ^.»op  a.oip 

5.55pl.05    5.1.5p  5.39p 

1.1.5a    .71  li).45p  10.56p 

6.45p   .35    C.20p  6.35p 


8.25a 
8.05p 
DN 


2.15 
3.61 


Aver. . . 
Great.. 


Duration. 

h.  m. 
1-35 
3-66 


1.16 
13.61 


C.20p 

5.54a 
5.43p 
10.45P 


6.14a 
6.30p 
11.30p 


Duration. 

h.  m. 
0-33 
0-45 


T 

T 
T 

.03 

.11 

1.87 

.30 
.10 
.30 
.30 
.14 

.13 
.09 
.18 

.50 
..30 
.65 
.39 
.31 

.36 
.31 
.60 

..35 
.90 
.43 
.35 

.39 

.38 
.86 

LOO 

.48 

.40 

1.03 

I'Oo 

.50 

.53 

1.18 

;75 

1.33 

!87 
1.46 

ioo 

1.49 

!92 
1.55 

L53 

1.61 

" 

.35 

1.87 

.17 
.30 

.40 
.65 

.51 
.90 

7'' 

.83 
1.18 

1.04 
1.32 

1.16 
1.46 

1.20 
1.49 

1.24 
1.55 

1.53 
1.53 

1.61 
1.61 

October. 


1897. 12 
1900.    8 
19a3.    8 

5.35a    10.05a 

12.45p      1.35p 

9.10a      2.06p 

1.93    6.59a      8.00a 

.40  12.54p      1.05p 

1.07    9.11a      9.16a 

.14 
T 
T 

.17 
.20 
.25 

.36 
.36 

.31 

.38 

.48 

.48 
.48 

.53 

.53 
.53 

.67 

.67 
.67 

.83 

.82 
.82 

.93 

.92 
.93 

1.05 

1.05 
1.06 

1.21 
1.21 

1.35 

1.35 
1.35 

1.45 

1.46 
1.45 

V 

Aver.. 
Great. 

Duration. 

h.  m. 
3-25 
4  55 

1.13 
19.2 

Duration. 

h.  m. 
0-28 
1-01 

.06 
.14 

.21 
.26 

.31 

.38 

.34 

.33 

•  Sept.  6, 1895,  rain  began  2.10  a.  m.,  ended  6.45  p.  m..  amount  4.76  inches.  The  gauge  was 
not  working,  hence  onlj'  stick  measurements  were  possible,  and  it  is  estimated  that  1.06 
inches  rain  fell  between  4.00  and  6.00  p.  m. 


208 


THE    CLIMATE    OF   BALTIMORE 


TABLE  LI  CONT.— EXCESSIVE  RATES  OF   RAINFALL  IX  CUMULATIVE 
FIVE-MINUTE   PERIODS. 


u 

P 

Total          a 
duration.       es 

|2 
Excessive      o  ^ 
rate.          || 

Excessive  periods  in  minutes 

Begin- 1  End-  | 
ning.       ing.    p 

Begin- 
ning. 

End-  la© 
ing-    i*^ 

5    10  j  15 

20 

25 

30    35    40  1  45    50 

60    80 

November. 

1896  '^^''   ''  ^^^  '  ^  ^^"  i  Q.ti     3  nnn 

4.00p   .03 

_.  i  .              _. 

.93 

1         ! 

i 

Table  LI  contains  a  complete  list  of  all  occurrences  of  excessive  rainfall 
from  1894  to  1903,  arranged  according  to  months  and  years.  The  scale 
of  excessive  rates  of  precipitation  adopted  by  the  U.  S.  Weather  Bureau, 
and  employed  in  the  above  classification,  is  shown  in  the  text.  The  above 
table  shows  the  year,  month  and  day  of  occurrence  of  the  excessive  rainfall, 
the  time  of  the  beginning  and  ending  of  the  entire  rain  period,  and  also 
of  the  period  of  excessive  rate  of  fall,  the  total  amount  of  fall,  and  the  cumu- 
lative amounts  which  fell  in  the  successive  5-minute  periods.  The  symbol 
{ \/)  in  the  last  column  indicates  the  occurence  of  a  thunderstorm  in  con- 
nection with  the  rainfall;  •.•  indicates  the  occurrence  of  a  thunderstorm  on 
the  same  day  at  some  other  hour. 

details  of  every  instance  of  excessive  rate  at  Baltimore  occurrintj  since 
1893,  at  which  time  the  automatically  recording  raingage  was  installed 
at  the  Baltimore  office. 

The  arrangement  is  by  months  and  years  and  shows  the  following 
facts:  the  year,  the  day  and  hour  of  occurrence,  the  total  amount  of  the 
rainfall  during  the  entire  progress  of  the  storm,  the  time  of  beginning 
and  ending  of  the  excessive  rate  of  fall,  the  amount  of  rainfall  before 

TABLE  LII.-SDMMARY  OF  EXCESSIVE  RATES  OF  PRECIPITATION  IN 
CUMULATIVE  FIVE-MINUTE  PERIODS. 


E 

xcessive 
rains. 

o® 

V 

Excessive  periods. 

d 

Dura- 
tion. 

Am'ts. 

5      10 

15 

20     25    30    35      40        45 

1 

50 

1 
00  ' 

Averages. 


January 

March 

May 

June 

July 

August 

September. 
October  — 
November  . 

Average  — 


h.  m. 

in. 

min. 

1 

12-35 

1.35 

22 

.05 

.10 

.25 

.45 

1 

0-40 

.34 

6 

.01 

.26 

.29 

la 

3-45 

1.(12 

19 

.23 

.40 

.51 

..56 

.6'; 

.83 

.94 

1.51 

1..T4 

i:{ 

4-23 

.75 

18 

.21 

.35 

.46 

.."17 

.fit- 

.78 

l.H 

1-56 

i.on 

21 

.35 

.m 

.74 

.88 

1.2S 

1.43 

1.62 

1.23 

.94 

.95 

1.20 

1. 

30 

2-48 

1.09 

24 

.15 

.35 

..51 

.65 

.85 

1  23 

1.25 

1.13 

1.05 

.91 

1.06 

8 

1-35 

1.16 

92 

.27 

.40 

.51 

.72 

.82 

1.04 

1.16 

1.20 

124 

l.5h 

1.61 

3 

3-25 

1.13 

36 

.21 

.31 

.34 

.48 

.52 

.67 

.82 

.93 

1.05 

1.21 

1.35 

1 

1 

3-10 

.98 

.93 

78 

3-49 

.98 

20 

.16 

.38 

.45 

.62 

.79 

1.00 

1.16 

1.20 

1.16 

1.16 

1.23 

1 

80 


45 


:marylaxd  weather  service 


209 


TABLE    LII    CONT.- 


-SUMMARY    OF   EXCESSIVE    RATES    OF    PRECIPITATION    IN 
CUMULATIVE    FIVE-MINUTE    PERIODS. 


Excessive 
rains. 

Excessive  periods. 

50 

60 

Dura- 
tion. 

Am'ts. 

Si 

5      10 

15 

20 

25    30    35      40        45 

SO 

Greatest. 


Ih.  m.  I  in.  |h.  m. 

January 12-35  1.35  0-22 

March 0-40     .34  0-6 

May 11-55  1.63  0-37 

June 15-45  1.21  0-30 

July 6-47  2.87  1-20 

August 12-50  2.05  0-59 

September 2-55  3.61  0-45 

October 4-55  1.92  1-01 

November 3-10     .98 


.05 
.01 
.57 
.35 
.37 
.46 
.30 
.25 


Greatest. 
Year 


Month 
Day.... 


Hour  of  beginning.. . 


15-46 


2.87  1-20 


.57     .98 


.46 


1.7"  2 
L161 

.901 

.36 


,80 
.232 
,36:1 
.021 


1.22 


1.722.2312.522 


1.44  1.51     1.64 


1.39 
1.46 

82 


s  : 
i  "^ 

1.49p 


1903 
July 

12 

12.04  p.  m. 


1.94 

1.51 

1.49 

.92 


1.94 
1901 


26 


1.04 
1.56 
1.55 
1.05 


1.56 
1899 


.951.20 
1.001.06 
1.5S!1.61 
1.211.35 

..  I  .93 

1.581.61 

1900 

Sept. 

15 


1.80 
l!45 


1.80 
1896 


n 


6.03p  H.15p   10.45p    9.19p 


Table  LII  contains  a  summary  of  facts  contained  in  Table  LI. 

the  excessive  rate  began,  and  the  amounts  recorded  in  cumulative  five- 
minute  periods  during  the  continuance  of  the  excessive  rate.  When  the 
rain  fell  in  connection  with  a  thunderstorm  this  fact  is  also  noted.  It 
is  a  matter  of  record  that  in  almost  every  instance  these  excessive  rain- 
falls occur  in  connection  with  thunderstorms.  In  Table  LII  there  is  a 
summary  of  the  preceding  table,  containing  the  average  amounts  recorded 
during  cumulative  5-minute  periods,  and  also  the  greatest  amount  for 
the  same  intervals  during  the  entire  period  of  ten  years.  The  average 
and  maximum  rates  of  precipitation  are  also  shown  graphically  for  the 
entire  year  in  Fig.  58. 

Excessive  rates  of  rainfall  as  defined  by  the  U.  S.  Weather  Bureau,  and 
employed  in  its  published  records,  are  indicated  in  the  following  table : 

TABLE  OF  RATES  CONSIDERED  EXCESSIVE. 


Amount. 

Time. 

0.23 

Inch 

In      5 

minutes. 

0.30 

'•    10 

•• 

0.35 

"    15 

•' 

0.40 

••    liO 

•• 

0.45 

'•    25 

" 

0.50 

•'    30 

•• 

0.55 

'•    35 

•• 

Amount. 

Time. 

0.60   inch   in 

40 

rtJinutes. 

0.G5      " 

45 

0.70      " 

50 

0.75     " 

60 

0.80     " 

80 

0.90     " 

100 

1.00     " 

120 

210 


THE    CLIMATE    OF    BALTIMORE 


An  inspection  of  Table  LI  will  show  that  excessive  rates  of  fall  as 
defined  above  are  confined  almost  entirelv  to  the  warm  months  of  the 


10 

20 

0 

40 

50               60                 80               100         MiN. 

^ 

\ 

Inches 
2.50 

/ 

/ 

\ 

/ 

// 

\ 

2.O0 

/ 

\ 

■/ 

/ 

1 

I 

1-50 

// 

/ 

\ 

) 

y 

l\ 

7 

\ 

A 

y^ 

X 

/ 

/ 

X 

^^c 

■~-«.\__ 

^.^ 

r.oo 

\ 

/ 

7 

r 

y 

y' 

^ 

0.50 

/ 

^y 

/ 

D 

--- 

/ 

^>^ 

^ 

0 

^ 

Fig.  58a. — Excessive  Rates  of  Rainfall. 

A.  The  curve  A  represents  the  maximum   rate  of  precipitation  attained  in  any  con- 
secutive 5,  10,  15,  etc.,  minutes  during  the  heavy  rainstorm  of  July  12,  1903. 

B.  Represents  the  rate  during  the  first  5,  10,  15,  etc.,  minutes  after  the  beginning  of 
the  excessive  precipitation  of  the  storm  of  July  12,  1903. 

C.  Represents  the  average  rate  in  78  cases  of  excessive  rates  of  fall. 

D.  Represents  the  lower  limits  of  rates  considered  excessive  by  the  U.   S.   Weather 
Bureau. 

year.     Of  a  total  of  84  instances  recorded  in  eleven  years,  the  seasonal 
distribution  is  as  follows : 

Jan.      Feb.      Mar.     Apr.      May     June     July     Aug.     Sept.     Oct.      Nov.      Dec.      Year 
10  1  0  1.3  14  19  21  9  5  1  0  S4 

Over  90  per  cent  of  all  occurrences  are  credited  to  the  months  of  May 
to  September.     None  have  been  recorded  in  February,  April  and  Decem- 


MARYLAXD    WEATHER    SERVICE 


21] 


ber,  while  January,  March  and  Xovember  have  but  one  each.  Over  90 
per  cent  of  all  excessive  rains  have  occurred  in  connection  with  thunder- 
storms in  the  immediate  vicinity  of  the  station  of  observation.  The 
average  rate  of  fall  based  upon  all  excessive  rains  during  a  period  of  10 
years  is  indicated  by  the  following  figures  for  the  five-minute  cumulative 
intervals  from  5  to  80  minutes. 


E 

F  ^*«s,„^ 


10  20  30  40  50  60  80  100        MiN, 

Fig.  58b. — Excessive  Rates  of  Rainfall. 

E.  Represents  the  rate  of  precipitation  per  hour  during  the  heaviest  5,   10,   15,  etc., 
minutes  of  rainfall  in  the  storm  of  July  12,  1903. 

F.  Represents  the  average  rate  per  hour  for  78  cases  of  excessive  rates  of  fall. 

AVERAGE   AMOUNTS   OF   EXCESSIVE   RAINFALL. 
(In  inches  and  hundredths.) 

Number  of  minutes  from  beginning  of  excessive  rate. 
5        10        15       20       25       30       35       40       45       50       60       80 

Amount  of  fall Ki      .33      .45      .02      .79    1.00    I.IG    1.20    1.16    1.16    1.23    1.62 

Selecting  from  Table  Lll  tlic  maximum  rate  of  fall  for  each  of  tbe 
periods  indicated,  irrespective  of  the  month  in  which  it  occurred,  we 
have  the  figures  below,  whicli  represent  the  maximum  observed  rates  for 
the  first  5,  10,  15,  etc.,  minutes  after  the  beginning  of  the  excessive 
rate  of  fall. 
15 


212 


THE    CLIMATE    OF    BALTIMORE 


MAXIMUM  RATES  OF  RAINFALL. 


Minutes. 


During  first ;      6 

Amount  of  fall i  .57 

Month May 

Tear l  1897  [ 


15 
1.72 


20  I      25 
2.23     2.53 


30 


36 

2.87 


July 
1903 


40 
1.94 
July 
1901 


45 
1.56 
Aug. 
1899 


50        60         80 
1.58     1.61  ,  1.80 


September 
1900 


July 
1896 


Amount. 

Rate  per  hour. 

0.80 

Inches. 

9.60 

inches. 

1.3.5 

8.10 

" 

1.92 

•' 

7.68 

" 

2.32 

" 

6.96 

" 

2.58 

" 

6.19 

" 

2.75 

" 

5.50 

" 

2.87 

" 

4.31 

" 

2.87 

" 

3.44 

" 

2.87 

2.87 

" 

2. ST 

•• 

1.44 

" 

The  destructive  storm  of  July  12,  1903,  which  swept  over  Baltimore 
and  vicinity,  was  accompanied  by  a  downpour,  the  rate  of  which  was 
probably  never  equaled  in  the  annals  of  Baltimore  weather.  The  rate 
of  precipitation  as  measured  at  the  local  office  of  the  U.  S.  Weather 
Bureau  is  indicated  by  the  following  figures,  and  graphically  shown  in 
Fig.  58: 

RATE  OF  RAINFALL  IX  STORM  OF  JULY  12,  1903. 
For  any  period  of 

5   consecutive  minutes 0.80   inches 

10  "  •'       

15  '■  ■•       

20  "  "       

25  "  '•       

30  "  "       

40  "  "       

50  "  "       

60  "  "       

120  "  "       

Table  LII,  Maximum  Cumulative  5-^linute  Periods,  shows  a  different 
value,  namely,  the  precipitation  of  the  first  period  of  5,  10,  15,  20,  etc., 
minutes  after  the  he  ginning  of  the  excessive  rate  of  fall.  The  storm  of 
July  12,  1903,  showed  a  maximum  rate  for  every  period  from  5  minutes 
to  one  hour. 

A  rough  calculation  has  been  made  of  the  amount  of  water  which  fell 
within  the  limits  of  Baltimore  City  during  the  storm  of  July  12,  1903. 
It  was  probably  one  of  the  most  severe  storms  ever  witnessed  in  the  city. 
While  the  area  of  destruction  in  this  type-  of  storm  is  fortunately  of 
extremely  limited  extent,  it  may  be  safely  assumed  that  practically  all 
of  the  city  had  a  rainfall  approximately  equalling  the  amount  recorded 
by  the  official  gage.  The  central  path  of  the  storm  was  about  a  mile  dis- 
tant from  the  office  of  the  Weather  Bureau ;  nearer  the  center  of  the  path 
the  rainfall  was  probably  heavier  than  in  portions  of  the  city  beyond  its 
area  of  destructive  winds.     Hence  we  may  assume  the  officially  recorded 


MARYLAND    WEATHER    SERVICE 


213 


fall  to  be  a  safe  estimate  of  the  average  for  the  entire  city.  Assuming 
the  area  of  Baltimore  City  to  be  31.15  square  miles,  we  may  readily 
compute  the  amount  of  water  which  fell  during  the  progress  of  the  storm : 


WEIGHT  OF  RAINFALL  IX  STORM  OF  JULY  12,  1903. 
(Within  the  limits  of  Baltimore  City.) 

Depth  Gallons 

per  acre. 


o(  fall. 


During  the  first 

5   minutes 33  inch. 

10  ■•        98  " 

15  ••        1.72  " 

30  "        2.69  " 

35  "        2.87  " 

During  the  heaviest 

5-minute  fall    0.80  " 


7,466 
22,172 
38,913 
60,859 
64,931 

18,099 


Tons  within 
City  Limits. 

745.428 
2,213,696 
3,885,162 
6,076,369 
6,482,967 

1,807,098 


Frequency  of  Consecutive  Days  with  Eain  or  Snow. 
In  a  large  percentage  of  instances  when  rain  or  snow  falls,  it  is  con- 
fined to  a  single  day.  The  exact  percentage  depends  largely  upon  what 
is  regarded  as  a  day  with  rain.  Including  a  light  sprinkling  rain,  or  a 
flurry  of  snow,  in  our  calculations  we  find  that  in  the  past  33  years,  the 
precipitation  was  confined  to  one  day  in  36  per  cent  of  the  total  number 
of  days  with  rain  or  snow.  Considering  only  measurable  quantities  of 
precipitation  (0.01  inch  or  more)  the  percentage  is  increased  to  45.  In 
28  per  cent  of  all  cases  the  rain  or  snow  extended  into  two  consecutive 
days,  and  in  16  per  cent,  three  days,  when  we  take  account  of  "  traces." 
Counting  only  appreciable  quantities,  the  percentages  are  respectively 
31  and  13.  The  instances  of  precipitation  on  more  than  three  days 
decrease  rapidly  with  each  successive  day  added.  In  the  table  below, 
the  frequency,  excluding  "  traces "  of  rain  or  snow,  is  shown  for  each 
month  and  for  the  entire  year  to  the  maximum  period,  namely,  14  days. 
frequency  of  consecutive  days  with  rain  or  snow. 

(Expressed  as  percentages  of  the  total  number  of  cases  of  appreciable  rainfall  in 

33  years.) 


Days. 


Jan. 

Feb. 

42 

41 

3.5 

35 

13 

13 

7 

8 

O 

1 

2 

1 

1 

1 

"i 

Mar.  Apr. 


88 
33 
18 
6 
4 
1 
1 
1 


May   June '  July   Aug.  Sept.   Oct.   Nov. '  Dec.  Year 


10 


45 

31 

13 
6 
3 
1 

0.4 
0.1 
0.2 
0.1 
0.0 
0 

0.1 
0.0 


214 


THE    CLIMATE    OF    BALTIMORE 


Inchiding  days  with  ''  traces  "  of  precipitation,  the  annual  frequency 
is  indicated  by  the  following  figures : 

ANNUAL  FREQUENCY  OF  COXSECUTiyE  DAYS  WITH  RAIN  OR  SNOW. 
(Including  "traces.") 


Number  of  days. . .      1 

2         3      4    1  5 

6    1      7 
2.5  0.9 

8    1      9    !    10 

11 

13        13       14 

Percentage  of  pos- 
sible occurrence.    36 

I 
28         16       9       5 

0.9 

0.4      0.2 

0.3 

0.1      0.0      0.1 

In  the  past  33  years  there  has  been  no  single  instance  of  rain  or  snow 
on  more  than  1-i  consecutiye  days,  and  but  few  in  which  the  rain  occurred 
on  more  than  six  consecutiye  days.  There  haye  been  longer  periods  of 
"  unsettled  weather,"  but  these  will  be  found,  upon  inyestigation,  to 
haye  been  interrupted  by  one  or  more  days  without  eyen  a  "  trace  "  of 
rain.     (See  Table  LIV,  Wet  Spells.) 

Dry   Spells. 

While  the  rainfall  is  quite  eyenly  distributed  throughout  the  year, 
there  are  at  times  periods  of  many  days  without  appreciable  precipitation, 
or  at  least  of  amounts  insufficient  for  the  requirements  of  plant  growth. 
During  some  seasons  of  the  year  this  scarcity  of  rain  is  of  comparatiyely 
little  importance;  during  periods  of  critical  crop  growth,  howoyer,  it 
becomes  a  question  of  serio'us  moment.  What  constitutes  a  dry  spell  is 
largely  a  matter  of  arbitrary  judgment.  It  is  not  alone  the  number  of 
days  without  rain ;  pre-existing  conditions  enter  largely  into  the  problem, 
as  well  as  the  stage  of  deyelopment  of  yegetation.  In  the  classification 
of  dry  spells  noted  in  Table  LIII,  the  selection  includes,  as  a  lower 
limit,  all  periods  exceeding  two  weeks  during  which  the  precipitation  was 
less  than  one-tenth  of  an  inch.  While  this  limit  is  a  purely  arbitrary 
one,  a  period  of  14  days  with  either  no  rain  or  less  than  one-tenth  of  an 
inch  is  a  long  interyal  considering  the  ayerage  frequency  of  rains  in  this 
vicinity.  For  periods  longer  than  two  weeks,  a  proportionately  larger 
quantity  of  rainfall  was  allowed,  keeping  in  yiew  the  desire  to  select  only 
such  dry  spells  as  fell  yery  far  short  of  the  normal  quantity  of  rain  for 
the  dry  interyal.  In  the  course  of  33  3^ears  there  haye  been  58  periods 
answering  the  requirements  of  the  definition,  ayeraging  a  little  less  than 
two  per  year.  A  drought  of  this  character  cannot  be  regarded  as  seyere, 
but  it  may  be  followed  by  considerable  loss  to  the  farmer  or  trucker 


:marylaxd  weather  service 


215 


during  certain  critical  periods.  Ordinarily  there  are  from  ten  to  twelve 
days  per  month  with  rain  to  the  extent  of  .01  inch,  giving  a  ratio  of  one 
day  with  rain  to  two  without.  These  are  not  uniformly  distributed 
through  the  month,  but  are  very  likely  to  occur  in  groups  of  two  or  three 
days. 

The  total  monthly  frequency  of  periods  of  this  class,  together  with  the 
average,  the  maximum  and  the  minimum  number  of  days  included,  is 
shown  in  the  following  tabular  statement : 

DRY   SPELLS   IN  33  YEARS. 
I  With  less  than  one-tenth  of  an  inch  of  rainfall  in  two  weeks  or  more.) 


Jan. 

Feb. 

Mar. 

Apr. 

May  June 

July  Aug.' Sept. 

Oct. 

Nov.  Dec. 

Year 

Total  frequency 

" 

3 

3 

3 

7 

3 

3         3 

6 

12 

5          8 

58 

Max.  duration.. 

Min. 

Aver. 

21 
16 
18 

31 
18 

22 

31 
19 
26 

22 
U 
19 

36 
14 
21 

29 
20 
25 

25 
23 

45 
23 
30 

34 
17 
25 

51 
15 

27 

48 
14 
33 

51 
20 
29 

51  days 
14     " 
25     " 

Aver.  rainfaU  .. 

.05 

.17 

.12 

.15 

.11 

.21 

.10 

.28 

.12 

.20 

.17 

.30 

.18  inch 

J.. 

18 
1        1        1       1 

75                             18 
1        1      1        1 

8C                             18( 

— r-i     ,     1 

5                             18 
1        1       1       1 

)0                             18 

— 1 — 1 — i— r 

1 

)5                             19C 

-Till 

0 

1     1   ■]     I 

1 

Fts 

1 

1 

1 

MC" 

ll 

1 

APn 

1 

1 

1 

»,. 

1      1 

1 

1             1 

1 

JUVt 

1 

1 

Juit 

1 

1 

Ayr, 

1 

1 

1 

St>.T 

1 

1 

1 

1             1 

1 

Oct 

1 

ll 

1  1 

1 

III               1     1 

Nov 

1 

1 

1                    II                     1 

Otc 

1       ,        ,        , 

II,    , 

\l 

1    1    :    1 

III 

ill 

Fiu.  59. — Dry  Periods. 
The  diagram  shows  the  frefjuency  of  occurrence  and  the  seasonal  distribution  of  all 
periods  of  two  weeks  or  longer  with  a  precipitation  of  one-tenth  of  an  Inch  or  less  from 
1871   to  1004.     The  length  of  the  period  Is  roughly  shown  by  the  length  of  the  heavy 
vertical  lines.     The  exact  duration  of  the  period  is  shown  in  Table  LIII. 


216 


THE    CLIMATE    OF    BALTIMORE 


TABLE  LIII— DRY  SPELLS. 


Tear. 


!2;b 


1871. 
1872. 
1873. 

1874. 
1875. 

1876. 

1877. 
1878. 
1879. 
1880. 

1881. 
1882. 
1883 

1884. 
1885. 

1886. 
1887. 
1888. 
1889. 
1890. 


31 


9 
.23 
18 


0.25 
23 


SI 
.08 
21 

17 
.01 
14 


.03 

ir 


.29 
29 


.02 
19 


35 
.04 
16 


SO 
.18 
14 


i 
.23 

28 


.05 
15 


.3: 
20 


.24 
22 


U 
.05 
20 


5 
.17 
34 


9 
.16 
33 


SO 
.17 
19 


9 
.04 
17 


6 
.10 
23 

20 
.30 
36 

2i 
.59 
38 


.05 
19 


21 
.39 
51 


.27 
36 


16 

.10 
21 


17 
.03 
20 

29 
.06 
^3 


9 
.66 
41 
18 
.31 
22 


is 

1 
3 
1 

2 
3 
1 
2 
1 


MARYLAND    WEATHER    SERVICE 


217 


TABLE   LIII    CONT.— DRY   SPELLS. 


Year. 


189L 
1892. 
1893. 
1894 

1895. 

1896. 
1897. 

1898. 
1899. 
1900. 


1901 J. 

1902 ■! 

1903 < 

No.  of  dry  spells. 


2S 
.04 
16 


20 
.06 
31 


20 
.12 
31 
1 
.16 
19 


.09 
27 


.36 
36 


18 
.07 
23 


.26 
38 


.13 
20 


.10 
16 


7 
.10 
20 


12 


3 
.26 
22 


SS 

.0 

14 


.13 
39 


.27 
29 


S7 
.09 
23 


51 


^i; 


58 


Table  LIII  contains  a  list  of  all  of  the  more  pronounced  dry  periods 
occurring  near  Baltimore  from  1871  to  1903.  No  strict  definition  of  a  dry 
spell  has  been  adhered  to  in  the  selection  of  these  periods,  but  the 
table  contains  all  periods  exceeding  two  weeks  during  which  the  precipita- 
tion amounted  to  less  than  one-tenth  of  an  inch.  The  length  of  the  dry 
spell  in  days  is  Indicated  by  the  figures  in  heavy  face  type,  the  date  of  ending 
by  italic  figures,  and  the  total  precipitation  during  the  period  by  roman 
figures. 

These  dry  spells  are  most  frequent  in  the  month  of  October,  and  hence 
after  the  harvest  season.  Their  occurrence  in  May  has  been  compara- 
tively frequent.  Coming  at  a  time  when  soil  moisture  is  a  matter  of  the 
highest  importance,  the  dry  spells  of  this  period  are  serious  matters.    The 


218 


THE    CLIMATE    OF    BALTIMORE 


seasonal   distribution  of  these  periods   of  deficient  moisture  is  shown 
graphically  in  Fig.  59  for  each  year  from  1871  to  1903. 

In  another  classification  of  dry  spells,  all  periods  of  ten  or  more  con- 
secutive days  were  included  in  which  precipitation  was  less  than  .01 


— r  -r- 1   1 

1   I   I   1 

1 

1  1  I  1 

1  1  1  1 

1  I  1  1 

1 

1  1  1  I 

1 

T-r  1   1 

1 

1 

1  i 

II 

1 

1 

1 

1 

II 

II 

1 

II 

1 

II 

1 

1 

1 

1 

1     1 

1 

1 

1  1 

h 

ii 

11  1 

1 

1         1 

1 

1 

1 

II      1 

1   1 

II 

III 

II 

III 

1  1  I 

1. 

1  il 

1 

iliiii 

II 

1 

1 

il 

II  1 

III 

II   1 

1 

1  1 

liilll 

1 , . 

,1,, 

1  1  1  1 

1 

l<  ,1, 

1  1  1  1 

1  1  1  1  1 

1    1    1    1 

Fig.  60.— Dry  Periods. 

The  diagram  shows  the  frequency  of  occurrence  and  the  seasonal  distribution  of 
periods  ^ith  less  than  one-hundredth  of  an  inch  of  precipitation  in  ten  days  or  more 
from  1871  to  1004.  The  relative  length  of  the  period  is  approximately  shown  by  the 
length  of  the  heavy  black  vertical  lines. 

inch.     The  total  number  of  such  periods,  with  the  average  and  greatest 
duration,  is  shown  in  the  following  figures: 

DRY   SPELLS   IN  33  YEARS. 
(With  less  than  one-hundredth  of  an  inch  of  rain.) 


a 

•-s 

4 

14 
18 

fa 

6 

13 
14 

s 

3 
11 
11 

< 

9 
12 
14 

6 
14 
21 

a 

11 
12 

<-> 

1 

4J 
0. 

o 

O 

O 

> 

o 

Year 

Total  number 

8 

12 
18 

8 
11 
12 

17 
12 

22 

17 
13 
19 

12 
14 
29 

5 
14 
17 

103 
13 days 

og 

Greatest  du  ration 

:\rARYLAXD   WEATHER    SERVICE  219 

The  above  table  reveals  the  interesting  fact  that  the  longest  period 
experienced  by  Baltimoreans  in  34  years  without  rain  was  29  days.  This 
occurred  in  Xoveniber,  1874.  September  and  October  share  the  distinc- 
tion of  having  17  periods  each  of  this  class  of  dry  spells  out  of  a  total  in 
34  years  of  102.  The  average  duration  of  such  periods  is  only  13  days. 
These  facts  are  not  in  accordance  with  popular  impressions.  Scarcely  a 
summer  passes  without  some  reference  to  periods  of  five  or  six  weeks 
''  without  a  drop  of  rain."  These  statements,  upon  investigation,  are 
generally  reduced  to  "  no  rain  of  consequence."  The  dry  spells  of  this 
class  are  graphically  shown  in  Fig.  60.  There  seems  to  be  no  apparent 
periodic  grouping  of  these  periods  of  deficient  rainfall,  either  in  Fig.  59 
or  in  Fig.  60.  The  following  list  comprises  seasons  with  a  marked  defi- 
ciency in  rainfall : 

SEASONS  WITH  DEFICIENT  rRECiriTATION. 

(Departures  below  the  normal  in  inches  and  liiuiciredths.) 

Winter.                                  Spring.  Summer.                                Autumn. 

1829-30.  ..  .4.17                      1822 5.11  1844.... 4. o4  1819.... 4. 57 

18G4-65 4.70         1827 4.12  1849.... 4. 14  1825.... 4. 79 

1870-71 6.00         1845 4.46  1864 6.66  1842 4.22 

1871-72 5.73         1847 6.03  1869 7.(19  1863 4.32 

1900U1.  .  .  .4.80         1855 5.91  1870 4.85  1870 4.33 

1856 4.87  1893 6.69  1879 5.06 

1866.... 5. 85  1894 6.21  1884 5.23 

1869 4.49  1903 4.56 

1900 4.66 

Wet  Spells. 
While  rain  or  snow  storms  do  not  usually  exceed  two  or  three  days  in 
duration,  there  are  frequently  periods  of  much  more  extended  rainy  or 
unsettled  conditions.  The  more  conspicuous  "'  wet  spells  "  occurring  since 
1871  are  emiiiu'rated  in  Table  LIV,  which  contains  a  list  of  all  jioriods 
of  10  days  or  less  during  which  the  rainfall  or  snowfall  was  equal  to  or 
exceeded  the  mean  monthly  amount.  Longer  periods  were  included  when 
the  precipitation  was  proportionately  excessive.  The  last  day  of  the  wet 
spell  and  the  duration  in  days  are  indicated  in  the  table,  together  with 
the  total  amount  of  the  precipitation.  Such  periods  have  been  recorded 
on  an  average  of  less  than  two  times  per  year,  or,  more  accurately,  51 
times  during  33  years;  the  limits  of  variability  are  0  and  5.  They  occur 
in  all  months  of  the  year,  with  a  decided  preponderance,  however,  in  the 


220 


THE    CLIMATE    OF    BALTIMORE 


warm    months   of   July,    August   and    September.     Their   frequency   of 
occurrence  and  seasonal  distribution  are  shown  graphically  in  Fig.  61. 

One  of .  the  most  remarkable  periods  of  unsettled  weather  was  that 
accompanying  the  northeast  storm  of  April  19-25,  1901.  Rain  began 
early  in  the  morning  of  the  19th  and  continued  during  the  greater  part  of 


I    I    1    1 

1    1   1   1 

1 1  1 1 
1 

1  1 1  1 

1  1  1  1 

1  1  1  1 

1  1  1  1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1  1 

1 

1 

1 

1 

1 

h  . 

1 

1 

1 

.  ■  1   . 

1 

II 

1 

1 

h 

1 

1 

■ 

1 

1     1 

I 

1 1 

1     1 

1 

1 

1 

1    1    1    1 

1  ii 

1  I  1 1 

1  1  1  1 

1  1  1  1 

1  1  1  1 

1  1  1  1  _ 

Fig.  61.— Wet  Periods. 

The  diagram  shows  the  frequency  and  seasonal  distrilnition  of  periods  of  ten  days  or 
less  in  which  the  average  monlhJij  amount  of  precipitation  was  recorded.  The  amount 
recorded  is  roughly  indicated  by  the  length  of  the  heavy  vertical  lines  ;  the  exact  amounts 
and  the  length  of  the  periods  are  shown  in  Table  LIV. 

seven  days,  or  162  hours  between  the  beginning  and  ending  of  precipi- 
tation. The  rainfall  was  not  continuous,  however,  scattered  showers 
falling  on  April  21,  22,  23  and  25.  The  total  amount  of  rainfall  recorded 
was  2.03  inches,  not  a  large  amount  considering  the  great  duration  of 
the  storm.  Another  noteworthy  rainstorm  was  that  of  May  16-26,  1894. 
During  this  period  of  eleven  days  the  rainfall  was  scattered  and  at  times 


MARYLAND    WEATHEPx    SERVICE 


221 


heavy.     The  actual  duration  of  precipitation  was  ahout  63  hours, 
total  amount  recorded  durinij  the  entire  storm  was  ■i.45  inches. 


The 


TA.BLE  LIV.— WET  SPELLS. 


"" 5 

1873 } 

i 

1873 \ 

1874 -j 

1875 -j 

187fi -j 

1877 -j 

1878 ■/ 

1879 

1880 

1881 \ 

1882 j 

1883 -j 

1884 - 

i 

1885 "I 

1886 ^ 

1887 I 

1888 ■< 

1889 ^ 

1890 ' 


li 
11 
3.46 


19 
10 
4.61 


13 
5.4S 


19 
17 
3.71 

13 
11 
5.65 


52 
3.83 


29 
23 
6.39 


30 
6.31 


16 

4.25 

11 
11 
4.07 


i 
17 
7.63 


1 
7 
4.33 


23 
14 

S.81 


29 
9.61 


23 
4 
14.73 


31 
9 
5.20 


6.37 


11 
6.9S 


4 
4 
4.70 


26 
12 
5.76 


8 
5.60 


13 

n 

4.30 


9 
5.52 


fi 
4.19 


2i 

18 

10.10 

15 

10 
5.23 


27 

7 

4.61 


IS 

12 

4.53 


26 

5 

3.79 


4 

12 

1.09 


13 
3.39 


3.77 

S6 
21 
5.17 


222 


THE    CLIMATE    OF    BALTIMORE 


TABLE   LIV   CON'T.— WET   SrELLS. 


Tear. 


1891 

1893 < 

1893 \ 

1894 -j 

1895 } 

1896 ^ 

1897 \ 

1898 I 

1899 - 

1900 \ 

1901 ^ 

1902 

1903 < 

No.  of  wet  spells. 


9 
9 
5.13 


21 
11 
4,S1 


2S 
13 
4.55 


s 

8 
14.43 


16 
11 
4.0a 


■29 
13 

6.31 


20 
10 
5.98 


13 

19 

7.15 


4.S6 


t6 
6.03 


26  5 

5.29  4.11 


z"5 


[' 
[» 
[' 

ll" 

![' 
If- 

30  \\ 
34 


3      61 


Table  LIV  contains  a  list  of  all  of  the  more  pronounced  wet  spells  occuring 
near  Baltimore  from  1871  to  1903.  The  table  includes  all  periods  of  10  days 
or  less  during  which  the  average  monthly  amount  was  recorded.  Longer 
periods  were  included  when  the  precipitation  was  proportionately  excessive. 
The  last  day  of  the  wet  spell  and  the  duration  in  days  are  indicated  by 
figures  in  Italics  and  Roman  respectively;  the  bold  face  type  indicates  the 
amount  of  precipitation  recorded  during  the  stated  period,  in  inches  and 
hundreths. 

Table  LIV  comprises  the  more  conspicuous  wet  spells  of  the  past  34 
years  based  upon  excessive  amounts  of  rain.  Another  table  was  pre- 
pared, but  is  not  published  in  full,  in  which  the  basis  of  selection  is  the 
duration  of  unsettled,  rainy  weather.     It  includes  periods  of  six  or  more 


MARYLAXD    WEATHER    SERVICE 


233 


consecutive  days  with  a  "  trace  "  or  more  of  precipitation,  8  days  with 
not  more  than  one  day  without  rain  or  snow,  or  10  days  with  not  more 
than  two  days  without  precipitation.  ^More  extended  periods  were  in- 
cluded when  precipitation  occurred  on  two  in  each  successive  period  of 
three  days.  The  total  number  of  "  unsettled  periods ''  of  this  descrip- 
tion comprised  within  the  Si  years  is  164.  The  distribution  throughout 
the  year  is  indicated  in  the  following  summary;  the  last  line  indicates 
the  number  of  intervening  days  without  rain : 

PERIODS  OF  UNSETTLED  WEATHER. 
(Six  or  more  consecutive  days  with  rain.) 


Total  frequency  

Ma.xiraum  duration  in  days  

Number  of  davs  without  rain 

d 

03 
11 

19 
i 

15 

17 

4 

S3 

20 

23 

2 

a 
< 

14 
13 

1 

oj 

23 

23 

4 

c 

3 
►^ 

13 

18 
3 

"3 

1-5 

12 
15 

si 

3 
< 

17 

19 
4 

P. 
$ 

9 
12 

0 

§ 

7 
10 
0 

> 

o 

is 

11 

10 

1 

o 

P 

12 
6 

03 
<D 

164 
33 

4 

SEASONS  WITH  EXCESSIVE  PRECIPITATION. 


( Depai 

tures  above  the  normal 

in  inches 

and  bun 

Wi 

nter. 

Spring-. 

Summer. 

1823-24 . 

.  .  .5.95 

1820 6.89 

1817. 

.  .  .12.25 

1839-40. 

.  .  .4.90 

1839 7.59 

1820. 

.  .  .    4.55 

1858-50. 

.  ..9.95 

1851 4.90 

1829. 

.  .  .    6.87 

1865-66. 

...4.80 

1854 7.09 

1836. 

.  .  .    8.00 

1880-81 . 

.  .  .5.44 

1858 4.71 

1837. 

.  .  .    4.05 

1881-82. 

.  ..5.04 

1859 5.95 

1838. 

.  .  .    5.45 

1883-84. 

.  .  .4.51 

1863 6.13 

1846. 

.  .  .    5.62 

1901-02. 

.  ..4.83 

1890 10.34 

1856. 

.  .  .    5.02 

1902-03 

.  ..4.93 

1892 5.81 

1857. 
1885. 
1889. 
1891. 
1903. 

.  .  .    4.10 
.  .  .    4.12 
.  .  .    5.96 
.  .  .    4.84 
.  .  .    5.90 

Autumn. 

1821. .  . 

10.33 

1833 .  .  . 

4.00 

1843... 

7.35 

1854. .. 

9.13 

1873.  .  . 

4.13 

1876. . . 

6.22 

18r7. . . 

7.51 

1889. .  . 

5.33 

1902..  . 

7.92 

The  Distribution  of  Precipitation  in  Xormal,  Dky  and  Wet  Years. 

The  comparatively  uniform  distribution  of  precipitation  throughout 
the  year  in  the  vicinity  of  Baltimore  is  well  shown  in  Fig.  62  and  Fig.  63. 
In  Fig.  62  the  total  monthly  amounts  are  shown  for  the  dry  year  of 
1900,  when  but  31.57  inches  were  recorded,  12  inches  below  the  normal 
amount,  and  for  the  year  1889,  which  had  an  excess  of  nearly  19  inches. 
For  purposes  of  comparison,  a  normal  year  is  placed  between  the 
typical  dry  and  wet  years.     In  a  similnr  manner  llie  distribution  of  pro- 


224 


THE    CLIMATE    OF    BALTIMORE 


' 

5 
< 

u. 

Q 

Z 

o 
< 

5 

5 

":    .s 


4 

; 

s 

< 

s 

3 

D 

< 

<Si 

O 

z 

Q 

m 

__^j^ 

^ 

- 

^ 

- 

^ 

2 

- 

« 

" 

= 

\ 

■ 

^" 

«■ 

r 

1 

■■ 

= 

" 

; 

. 

- 

" 

J 

■■ 

z 

2 

1 

■" 

^ 

- 

- 

M 

2 

- 

^ 

- 

; 

•■ 

- 

ra 

— 

a 

- 

- 

Hi 

- 

= 

■ 

■■ 



- 

" 

- 

- 

a 

= 

■ 

= 

JJ 

■ 

■ 

mt 

- 

M 

m 

- 

: 

" 

- 

i 

- 

— 

- 

I 

■ 

hi 

01 

III 

> 

<A 

1- 

UJ 

(U 
^ 

? 

-tj 

>*    a 


o   — 
2   -z 


<A    -z 


ci 

' 

CJ 

>, 

>1 

Bi 

O 

>> 

jq 

Q 

1) 

rt 

<i-( 

tn 

c 

— 

;^ 

a 

0) 

0) 

C! 

— , 

X3 

>. 

03 

-.. 

<   ^  ■;; 

"J     r         S 


226 


THE    CLIMATE    OF    BALTIMORE 


cipitation  by  days  and  months  is  shown  for  the  same  years  in  Fig.  63.  The 
depth  of  rainfall  is  indicated  by  tlie  length  of  the  heavy  vertical  lines. 
The  dry  year  (1900)  was  deficient  in  rainfall  frequency  as  well  as  in 
amount.  The  normal  year  (1888)  had  154  rainy  days;  the  dry  year 
(1900)  had  115,  and  the  wet  year  (1889)  had  164.  The  rainfall  of  the 
dry  year  was  only  about  half  that  of  the  wet  year,  the  amounts  being 
31.57  inches  and  62.35  inches,  respectively.  The  normal  precipitation  is 
43.34  inches.  The  great  excess  in  the  wet  year  was  due  to  the  heavy 
spring  and  early  summer  rains  of  1889. 


TABLE  LV. 


-SUMMARY  OF  PRECIPITATION  DATA. 

(1871-1903.) 


January . 
February 

March 

April 

May 

June 

July 

August  — 
September 
October  .. . 
November. 
December . 


Means. 


3.20 

3.70 
3.99 
3.27 
3.63 
3.78 
4.6r. 
4.20 
3.8f) 
2.99 
2.99 
3.07 


Mean 
depart. 


Year 43.34 


J. 11 

1  47 
1.44 
1.19 
l.fil 
1.45 
2.03 
1.80 
1.83 
1.48 
1.12 
1.28 


5.36     12 


Monthlj'  and  annual  amounts. 


Greatest. 


Least. 


lA.     S     O 


1^    S 


fi.42  1892 

7.07  1896 
7.94  1891 

8.70  1889 

7.26  1894 

8.08  1883 
11.03  1889 

9.49  1873 

10.. ')2  1876 

fi.85  1902 

6.85  1877 

7.07  1901 

62.35I  1889 


201 
194 
199 
266 
200 
215 
241 
229 
267 
231 
224 
228 


0.88 
0.65 
1.19 
1.37 
1.00 
0.90 
1.40 
0.64 
0.09 
0.16 
0.65 
0.37 


1872 
1901 
1894 

1885 
1900 
1901 
1881 
1877 
1884 
1874 
1882 
1896 


144     31.. 57  !  1900 


No.  of  days* 
with  pre- 
cipitation. 


131  164     104 

in      in 

1 1889  1871 


h.  m 
7:18 
8:14 
6:1:^ 
6:25 
4:23 
2:45 
3:03 
2:42 
4:0:^ 

n-.m 

6:15 
6:46 

5:18 


Great- 
est in 
24hr8. 


1.951896 
3.481896 
3.511881 
3.581889 
2.991886 
4.471885 
4.021889 
4.361873 
4.761895 
.3.421^73 
2.851877 
2.881901 


4.76 


1895 


*  Omitting  days  with  only  a  "  trace  "  of  rainfall  or  snowfall. 

Table  LV  contains  a  summary  of  the  principal  facts  relating  to  precipitation 
and  published  in  full  in  preceding  tables.  The  first  column  of  figures  shows 
the  normal  monthly  precipitation  based  on  3.3  years'  observations;  the  second 
column  shows  the  proportion  of  the  annual  precipitation  which  falls  in  each 
month;  the  third  column  shows  the  average  amount  by  which  the  actual 
monthly  fall  differs  from  the  normal  monthly  fall,  either  above  or  below; 
the  next  following  column  of  figures  shows  the  same  fact  expressed  as  a 
percentage  of  the  normal  monthly  precipitation. 


marylaxd  weather  service  227 

Snowfall. 

There  are  many  difficulties  in  the  way  of  securing  accurate  measure- 
ments of  the  amount  of  snowfall,  difficulties  which  are  inherent  in  the 
conditions  attending  precipitation  in  general,  together  with  the  additional 
one  introduced  when  the  temperature  is  at  or  near  the  freezing  point  of 
water.  The  method  of  exposure  of  the  snow-gauge  is  of  highest  im- 
portance even  under  favorable  atmospheric  conditions  for  securing  all 
the  falling  snow.  When  the  wind  is  high,  and  especially  when  it  blows 
in  gusts,  the  snow  is  drifted  and  blown  about  to  such  an  extent  as  to  make 
it  impossible  to  catch  any  but  a  small  percentage  of  the  total  fall  in  the 
gauge.  Under  such  circumstances  it  is  necessary  to  resort  to  a  different 
method  of  measurement.  In  an  open  and  exposed  area  several  measure- 
ments are  made  of  the  actual  depth  in  inches  of  snow  on  the  level  ground 
at  points  which,  in  the  estimation  of  the  observer,  represent  most  nearly 
the  average  depth  in  the  vicinity  of  his  station.  The  average  of  thc^o 
measurements  is  then  accepted  as  the  true  depth  of  snowfall.  In  order 
to  secure  the  equivalent  depth  in  melted  snow,  the  several  measured  depths 
are  melted  and  the  average  depth  of  water  obtained  is  computed.  The 
amount  of  water  yielded  by  a  given  depth  of  snow  varies  greatly  with  the 
temperature  of  the  snow  and  the  conditions  under  which  it  falls.  A  light 
fluffy  snow  may  require  15  to  20  inches  for  one  inch  of  water :  on  the  other 
hand,  a  wet  soggy  snow  of  4  or  5  inches  may  melt  to  an  inch  of  water. 
In  rough  measurements,  under  average  conditions,  the  ratio  is  about  ten 
to  one,  and  this  is  the  relation  generally  assumed.  With  a  slight  change 
in  temperature  at  or  near  the  freezing  point,  the  snow  melts  as  it  falls, 
or  after  falling  for  some  time  it  may  change  to  rain.  These  are  some  of 
the  difficulties  encountered  in  an  effort  to  secure  reliable  snowfall  data. 

The  record  of  fairly  accurate  depths  of  snowfall  at  Baltimore  begins 
with  the  year  1883.  The  record  of  frequency  of  snowfall  begins  much 
earlier,  dating  from  the  opening  of  the  Weather  Bureau  Station  in  18T1. 
The  actual  monthly  and  seasonal  amount  of  snowfall  recorded  during 
each  month  and  year  from  1883  to  1904  is  shown  in  Table  LVI,  together 
with  the  monthly  and  seasonal  average  amounts  for  the  entire  period  of 
21  seasons.  The  seasonal  variations  are  shown  in  Fig.  6-1.  The  average 
16 


228 


THE    CLIMATE    OF    BALTIMORE 


u^     ^ 


_    o 


-    0 


in     S 

<1 


MARYLAND    WEATHER    SERVICE 


129 


230 


THE    CLIMATE    OF    BALTIMORE 


TABLE  LVI.-MONTHLY  AND  SEASONAL  SNOWFALL. 
(In  inches  and  tenths.) 


Season. 


1883-4.... 

1884-.").... 
1885-6... 


1886-7.. 
1887-8.. 
1888-9  . 
1889-90. 
1890-1.. 


1891-2. 
1892-3. 
1893-4. 

1894-5. 
1S95-6. 


1897-8.... 
1898-9  ... 
1899-1900. 
1900-1.... 


1901-2 

1902-3 

1903-4 

Average  (1884-1904). 

Greatest 

Year 


Oct. 

Nov. 

Dec. 

Jan. 

Feb. 

Mch. 

Apr. 

May. 

Sea- 
son. 

0 

0 

4.4 

14.2 

0 

8.7 

8.0 

0 

35,3 

0 

0.6 

3.8 

1.9 

17.2 

5.9 

2.0 

0 

31.4 

0 

0 

0 

13.0 

15.3 

2.0 

0 

0 

30.3 

0 

T 

10.2 

2.5 

6.0 

6.8 

1.1 

0 

25.6 

0 

0 

12.0 

8.8 

3.9 

7.4 

0 

0 

32.1 

0 

1.1 

T 

2.6 

5.3 

T 

T 

0 

9.0 

T 

0 

0 

0.1 

2.5 

2.3 

T 

0 

4.9 

T 

T 

10.6 

1.3 

3.5 

20.5 

0 

T 

35.9 

0 

T 

T 

14.5 

4.2 

25.6 

T 

0 

44.3 

0 

o  3 

4.3 

8.1 

11.7 

4.0 

0 

T 

30.3 

0 

0.2 

3.1 

1.0 

11.7 

T 

5.0 

0 

21.0 

0 

T 

3.0 

5.0 

9.3 

0.6 

0 

0 

17.9 

T 

T 

0.2 

1.0 

2.8 

13.8 

T 

0 

17.8 

0 

3.0 

3.2 

4.7 

0.7 

T 

0 

0 

11.6 

0 

T 

2.6 

5.4 

0 

2.4 

0.1 

0 

10.5 

0 

9.7 

0.6 

!.3 

33.9 

1.6 

0 

0 

.M.l 

0 

0 

0.7 

2.5 

13.0 

9.5 

T 

0 

25.7 

0 

T 

T 

6.5 

2.1 

0.1 

0 

0 

8.7 

0 

0.1 

0.6 

6.7 

1.0 

5.0 

0 

0 

13.4 

0 

0 

7.0 

6.8 

6.0 

0 

0 

0 

19.8 

T 

1.0 

3.8 

16.6 

2.5 

2.0 

0 

0 

25.9 

T 

0.8 

3.3 

5.6 

7.5 

5.8 

0.8 

T 

23.8 

T 

9.7 

12.0 

16.6 

33.9 

25.6 

8.0 

T 

51.1 

1898 

1887 

1904 

1899 

1892 

1.HH4 

1898-9 

depth  of  snow  recorded  during  each  month  and  the  greatest  and  least 
monthly  amounts  recorded,  are  as  follows: 


MONTHLY   SNOWFALL, 
fin  inches  and  tenths.) 


Average  . 
Greatest. 

Y''ear 

Least  — 
Year 


(1884-1904) 


Oct. 

Nov. 

Dec. 

Jan. 

Feb. 

March 

April 

May 

*T 

0.8 

3.3 

5.6 

7.5 

B.8 

0.8 

T 

T 

9.7 

12.0 

16.6 

33.9 

25.6 

8.0 

T 

1898 

1887 

1904 

1899 

1892 

1884 

0 

0 

0 

0.1 
1890 

0 
1898 

0 
1903 

0 

0 

Season 


23.8 
51.1 
1898-9 
4.9 
1889-90 


*  T  represents  a  trace  of  snow. 

February  is  the  month  of  greatest  snowfall,  followed  by  March,  with 
January  third  in  the  order  of  depth.  The  annual  fall  has  varied  from  a 
minimum  of  about  5  inches  in  the  season  of  1889-90,  to  a  maximum  of 
51  inches  in  1898-9.  Every  month  of  the  year  excepting  January  has  at 
some  period  since  1882  been  entirely  free  from  snow.  The  greatest 
monthly  snowfall  occurred  during  February,  1899,  when  about  3-i  inches 


MARYLAND    WP:ATHER    SERVICE  231 

were  recorded.     Over  half  of  this  amount  fell  during  the  great  blizzard 
of  that  month. 

Expressed  in  terms  of  the  percentage  of  the  total  annual  precipitation, 
the  average  annual  snowfall  at  Baltimore  is  5.6  per  cent;  that  is,  about 
one-eighteenth  of  the  amount  representing  the  total  annual  precipitation 
falls  in  the  form  of  snow.  The  percentage  has  varied  from  1  per  cent 
in  the  calendar  year  1889,  to  11  per  cent  in  1892.  Computing  the  rela- 
tive amounts  which  fall  as  snow  and  rain  in  the  season  of  snowfall  only, 
we  have  the  following  figures : 

RAINFALL  AND  SNOWFALL  OF  THE  WINTER  SEASON. 

(In  percentage  of  total  monthly  precipitation.) 

Rainfall.  Snowfall. 

November     97  per  cent.  3  per  cent. 

December     89     "       "  11     " 

January     83  "       "  17     " 

February      82     "       "  18     " 

March     8G     "        "  14     " 

April     98     "       "  2     " 

Average      89     "       "  11     " 

Even  in  the  mid-winter  months  of  January  and  February,  the  amount 
of  snowfall  is  generally  less  than  one-fifth  the  total  precipitation  for  those 
months. 

Dates  of  First  and  Last  Snow. 

The  first  snow  of  the  season  usually  falls  about  the  15th  of  Xovember, 
and  the  last  about  the  first  of  April ;  hence  the  average  length  of  the 
season  of  snowfall  is  about  four  months  and  a  half.  These  first  and  last 
snows  are,  however,  usually  only  light  fiurries.  This  is  particularly  true 
of  the  first  autumn  snows.  In  the  34  seasons  since  1871,  snow  flurries 
have  occurred  as  early  as  October  9,  as  in  1895  and  1903.  The  first  snow 
of  the  season  has  occurred  as  late  as  December  17,  as  in  1883  and  1887. 
The  early  snows  were  not  followed  by  either  an  abnormal  amount  or  by 
an  abnormal  frequency  of  snows.  The  last  snow  of  the  season  has  occurred 
as  late  as  May  6,  as  in  1891,  and  as  early  as  February  22,  as  in  1903. 
Table  LVII  contains  a  record  of  first  and  last  snows  for  each  season 
from  1871  to  1904. 


232 


THE    CLIilATE    OF    BALTIMORE 


TABLE  LVIL-DATES  OF  FIRST  SNOW  IN  AUTUMN  AND  LAST  IN  SPRING. 
(Including  "  traces  "  of  Snow.) 


Year. 


1871 

1872 

1873 

1874 

1876 

1876 

1877 

1878 

1879 

1880 

1881 

1882 

1883 

18^4 

1885 

1886 

1887 

1888 

1889 

1890 

1891 

1892 

1893 

1894 

1895 

1896 

1897 

1898 

1899 

1900 

1901 

1902 

1903 

1904 

Earliest. 
Latest .. 
Average 


First  in  Fall. 


Last  in  Spring. 


Nov 

29 

Mar.   4 

" 

29 

••        0'> 

" 

13 

"     21 

" 

13 

8 

Apr.    1 
^     18 

Oct. 

15 

Mar.    2 

Nov 

10 

"     29 

Dec 

5 

Feb.  25 

Nov 

5 

Apr.    5 

** 

13 

Mar.  29 

Nov 

24 

Apr.    4 

" 

26 

"     11 

Dec. 

17 

Mar.  31 

Nov 

3 

Apr.    9 

" 

23 

"     11 

Nov 

13 

Mar.    8 

Dec. 

17 

Apr.    5 

Nov 

24 

Mar.  25 

Oct. 

23 

Apr.    6 

" 

19 

"       1 

Nov 

28 

May    6 

" 

9 

Apr.  15 

" 

15 

May    4 

" 

30 

Apr.  12 

Oct. 

9 

Mar.  20 

Nov 

13 

Ai)r.    7 

" 

33 

Mar.  14 

" 

24 

Apr.  28 

Dec. 

4 

"     16 

Nov 

9 

4 

Nov 

18 

Mar.    6 

Dec. 

5 

"     31 

Oct. 

9 

Feb.  22 

Nov 

13 

Mar.  28 

Oct. 

9 

Feb.  22 

Dec. 

17 

May    6 

Nov 

15 

Apr.    1 

Year. 


1871 
1872 
1873 
1874 
1875 

1876 
1877 
1878 
1879 
1880 

1881 

1882 
1883 

1884 
1885 

1886 
1887 
1888 
1889 
1890 

1891 
1892 
1893 
1894 
1895 

1896 
1897 
1898 
1899 
1900 

190] 
1902 
1903 
1904 


The  Frequency  of  Days  with  Snowfall. 
The  frequency  of  days  with  snow,  including  "  light  flurries,"  varies 
greatly  from  year  to  year.  The  average  number  for  a  series  of  years  is, 
however,  fairly  constant.  Dividing  the  entire  j^eriod  of  30  years  from 
1871  to  1900  into  three  periods  of  ten  years  each,  the  average  annual 
frequency  was  as  follows : 

AVERAGE  ANNUAL   SNOWFALL  FREQUENCY. 
(Including  traces.) 

1871  to  1880 16.4  days. 

1881  to  1890 24.0   " 

1891  to  1900 26.1   " 

Mean  (1891  to  1903) 22.0   " 


MARYLAND    WKATIIER    SERVICE 
Oct.        Nov.  Dec.  jan.  Fes  Mch.  Apr.  Ma 


233 


Inches    30 


20 


\  A 

\ 

/ 

^ 

B 

\ 

"      ^^ 

^ 

^^ 

\; 

i^ 

c\ 

\ 

/ 

^ 

s. 

\ 

/^ 

N 

N  \ 

/ 

E      ~^^ 

k 

^ 

^ 

A 

30  Inches 


Oct.         Nov.  Dec.  Jan.  Feb  Mch.  apr  May 

Fig.  65. — Monthly  Frequency  and  Amount  of  Snowfall. 

A.  The  greatest  monthly  frequency  of  days  with  appreciable  snowfall. 

B.  The  average  frequency. 

C.  The  greatest  monthly  amounts  of  snowfall. 

D.  The  average  monthly  amounts  of  snowfall. 

E.  The  least  monthly  amounts  of  snowfall. 

With  an  average  seasonal  frequency  of  22.  the  numher  has  varied 
from  5  as  in  1875-6  to  40  as  in  1892-3.  The  average  monthly  and 
seasonal  frequency  for  34  seasons,  including  light  flurries  of  snow,  or 


234 


THE    CLIMATE    OF    BALTIMORE 


"  traces,"  is  indicated   in  the  following  table.     The  variations  in  the 
seasonal  frequency  are  shown  in  Fig.  65. 


FREQUENCY  OF  DAYS  WITH  SNOW. 
(Including  "snow  flurries.") 


Average 
Greatest 
Least — 


Oct. 

Nov. 

0.2 

l.r, 

3 

5 

0 

0 

4.1 
10 
0 


Jan. 

Feb. 

Mar. 

Apr. 

May   j  Season 

5.8 
18 
0 

5.6 
14 
0 

3.8 

8 

0 

0.8 

3 

0 

0.1     23  days 
1        140     •' 

0        j  5     " 

If  we  do  not  take  into  account  da3's  with  light  flurries  of  snow  but 
only  days  during  which  a  tenth  of  an  inch  or  more  fell,  or  days  with 


TABLE  LVIII.— NUMBER  OF  DAY^S  WITH  SNOWFALL  EQUALLING  OR 
EXCEEDING  0.10  INCH. 


Season 

Nov. 

Dec. 

Jan. 

Feb. 

Mar. 

Apr. 

Season 

1370  1              

'6 

5 
4 
0 

0 
6 
0 
1 
3 

6 
o 

0 
3 
3 

0 
9 
3 
0 
0 

8 
0 
3 

i 
1 

3 

1 
1 

0 
3 
3 
6 

3.0 
3.6 
2.2 

2.3 

3 

5 
2 
o 

3 

0 
3 
1 

1 
0 

4 
4 
9 
3 

4 
3 

6 

2 

1 

3 
4 

7 
3 

5 

1 
6 
3 
4 
3 

o 

3 
3 

8 

2.0 
3.8 
3.6 

3.1 

3 
3 

5 
5 
3 

2 
2 
1 
6 
3 

3 
3 
o 

6 

13 
3 

0 

3 

5 
3 

2 
3 

7 
8 
4 

3 
2 
0 
9 
4 

3 
1 
2 
4 

3.1 
3.5 

4.3 

3.4 

0 

5 
1 
0 
3 

3 

0 
3 

2 

0 
3 

3 

5 
6 

1 
3 
4 

0 

4 
6 
3 

0 
1 

6 
0 

1 
1 
4 

1 
2 
0 
3 

1.8 
2.6 
2.5 

3.2 

0 
0 
0 
1 
1 

0 
0 
0 
0 
0 

1 

1 

0 

1 

1 

0 
3 

0 
0 

1 

0 
0 
0 
3 
0 

0 
0 

1 

0 
0 

0 
0 
0 
0 

0.2 
0.7 
0.3 

0.4 

6 

1871  2  

0 

12 

187"  3  

0 

13 

1873  4                        

0 

13 

1874-5  

1875-6 

0 

0 

10 

4 

1876-7 

1877  8  

1 

0 

15 
3 

1878  9 

0 

10 

1879-bO 

1880  1                           .... 

0 

4 

6 
18 

1881  2              

0 

11 

188"  3  

4 

18 

188.3-4 

1884-5  

1885-6  

1886  7                 

0 

0 

0 

13 
26 

8 
19 

1887  8  

0 

16 

1888  9  

1 

8 

1889-90                      

0 

7 

1890  1  

0 

16 

1891-3 . . 

1893-3 

18^3  4                             

0 

4 

1 

13 
23 

15 

1894  5            

0 

11 

1895-6 

1896  7  

0 

1 

11 
11 

1897  8                                  •    . 

0 

8 

1898-9 .           

3 

18 

1899-1900  

1900  1  

0 

0 

11 
5 

1901  3                                .    . 

1 

9 

1903  3  

0 

7 

1903  4 

1 

23 

Average. 

1871-1880  

1881-1S90   

1H91  1900                          

0.1 

1.1 

0.9 

9.3 
14.3 
13.7 

1871  1903 

0.7 

13.0 

MARYLAND    WEATHER    SERVICE 


335 


what  may  be  reaardeil  as  ''  appreciable ''  snowfall,  the  monthly  and 
seasonal  frequency  is  reduced  considerably  below  the  figures  shown  in 
the  preceding  paragraphs.  A  detailed  list  of  such  days  is  contained  in 
Table  LYIII,  which  gives  a  more  satisfactory  index  of  the  snowfall  con- 
dition of  a  season  than  the  figures  which  include  "  traces."  Basing  our 
calculations  upon  "''  appreciable  "  snowfalls,  Ave  have  an  average  seasonal 
frequency  of  1"3  days.  The  season  of  18T7-S  contained  but  2.  while  26 
were  recorded  in  1884-5.  The  variations  in  the  seasonal  frequency  are 
shown  in  Fig.  65.  The  average  per  month  for  the  S-i  seasons  since  1871 
is  as  follows: 

FREQUENCY   OF  DAYS  WITH   SNOW. 
(Excluding  traces.) 


Nov.      Dec.       Jan.      Feb. 


Average.. . 
Greatest . 
Least 


2.2 

P 

0 


3.1 


3.4 
12 
0 


Mar.       Apr. 


2.2 

e' 

0 


0.4 
2 

0 


Season 


13.0  days 
26 


Heavy  Snowfalls. 

The  heaviest  snow  noted  in  the  oflficial  records  of  the  local  otHce  of  the 
Weather  Bureau  fell  during  the  great  "  blizzard  "'  of  February,  1899. 
The  fall  occurred  in  connection  with  an  Atlantic  coast  storm  which 
reached  Maryland  at  a  time  when  the  Middle  Atlantic  states  w^ere  in 
the  embrace  of  the  severest  cold  wave  of  the  past  30  years.  The  ground 
was  already  covered  by  snow  to  the  depth  of  about  10  inches,  which  fell 
from  the  5th  to  the  8th,  and  to  this  layer  5  inches  were  added  on  the 
12th  and  15.5  inches  on  the  13th.  At  the  close  of  the  storm  of  the  12th 
and  13th,  the  depth  of  snow  on  the  ground  measured  30  inches  in  the 
city  of  Baltimore.  Greater  depths  were  reported  from  other  parts  of 
Maryland.  The  wind  was  high  and  the  temperature  was  extremely  low, 
rancriutl  between  5°  and  20°  below  zero  within  the  state.  As  a  result, 
the  dry  snow  was  very  much  drifted  and  settled  in  places  to  depths  of 
10  to  20  feet.  The  city  was  snoAvbound  and  all  local  traffic  was 
blocked  for  two  or  three  days. 

The  greatest  depth  of  snowfall  for  any  24  consecutive  hours  during 


236 


THE    CLIMATE    OF   BALTIMORE 


this  storm  was  15.5  inches,  according  to  the  official  measurements. 
Single  snowfalls  equalling  or  exceeding  10  inches  in  2-i  hours  are  ex- 
tremely rare  in  the  vicinity  of  Baltimore.  There  was  one  on  December 
17,  1887,  another  on  the  3d  of  February,  1886,  and  another  on  the  18th 
of  March,  1892,  in  the  21  years  since  1884.  In  Table  LIX  will  be  found 
a  record  of  the  heaviest  2-i-hour  snowfall  for  each  month  and  season  from 
1884  to  190-i. 

TABLE  LIX.-GREATEST  SNOWFALL  IX  24  CONSECUTIVE  HOURS. 


Season. 


1883-4. 
1884-6. 


1885-6.. 
1886-7.. 
1887-8.. 
1888-9.. 
18^9-90. 


Oct. 


1890-1 T      19 

1891-a .. 

1892-3 1  .. 

1893-4 

1894-5 


189.5-6.... 
1896-7.... 
1897-8  ... 
1898-9.... 
1899-1900. 


1900-1. 
1901-2 
1902-3. 
1903-4. 


Greatest. 


Nov. 


Dec. 


Jan.       Feb. 


0.5 


2.9 

1.0 


0    0 

3.1     6 

10.6  17 

T     19 

0      0 


3.0  15 
1.0'  28 


T 
T 
1.2 
0.2  15 

T     30 

T  i  20 
3.0  30 


6.5     9  13.0  3 

1.5     5    4.0,  26 

3.2     9     1.8  13 

2.5j  20  I  2.3;  27 

G.li  23  I  1.5:  2 


T 

0.1  29 


1.0  29 


4. 511898 10. 6  IBS' 


3.0  26 

0.2  13 
2.3  22 

2.0  26 
0.6   12 

0.71  27 

T  I  21 
0.5  23 
4.0  5 
1.6    2 


1.0  25 

7.0  15 

4.8,  12 

0.5  27 

2.6|  29 

1.0  19 

3.6  27 

3.0  31 

2.8  1 

1.5  28 

4.0  25 

5.6  29 
4.0  34 
6.0,  29 


3.0  26 

4.0  6 

7.8!  17 

3.5!  25 

4.3|  7 

1.0!  i 
0.5  8 
T  25 
15.. 5,  13 
6.0  17 

I 
2.0    3 
1.0  17 
5.0  17 
1.6   19 


Mar. 

5.0 

5 

1.5 

13 

2.0 

8 

3.6 

4 

3.5 

5 

T 

7 

3.4 

31 

9.5 

27 

13.fl 

18 

3.6 

4 

'1' 

26 

0.6 

11 

April 


0.7 


6.0  11 

T  14 

2.4  2 

1.6  7 

4.5  15 

0.1 
4.0; 


1.5i  18 


.0!l892|l5.5!l899:i2.0l892 


8.0 


1884: 


May 


Season. 


8.0  Apr. 
5.5  Feb. 


13.0  Feb. 

4.0  Feb. 
10.6,  Dec. 

2.5!  Jan. 

3.4  Mar. 


9., 5  Mar. 
13.0  Mar. 
7.8!  Feb. 
4.0  Apr. 
4.3  Feb. 

6.0  Mar. 
3.0)  Nov. 
3.0  Jan. 

15.0!  Feb. 
6.0  Feb. 


4.0  Jan. 
5.6  Jan. 
5.0  Feb. 
6.0  Jan. 


15.5  Feb. 


The  first  column  shows  the  amount  of  snowfall,  and  the  second  the  date  of 

occurrence. 


Duration    of    Snowfall. 

An  effort  has  been  made  to  obtain  a  value  for  the  average  duration  of 
snowstorms  in  this  vicinity.  For  this  purpose  the  records  were  carefully 
examined  for  times  of  beginning  and  ending  of  snowfall  for  the  period 
from  1884  to  1889,  and  the  period  from  1893  to  1902.  Nq  g^eat  accu- 
racy can  be  claimed  for  the  results,  as  there  is  no  method  in  use  for 
automatically  recording  beginnings  and  endings  of  snowfall.  However, 
the  figures  given  are  based  on  a  tabulation  of  266  cases  of  snowfall  during 


MARTLAXD    WEATHER    SERVICE  237 

TABLE  LX.-SUMMARY  OF  SNOWFALL  DATA. 


Means. 

■S 

OD 

_j- 

.Mii- 

5 

^ 

C  O. 

o    .       ®_ 

=  5   ,   =i  =i 

<»2   1   t.3 

^  ^^ 

o  z 

Greatest 
monthly  am'ts. 


Number  of  clays  with  snow. 


Greatest 

snowfall 

in  24  hours. 


1871-1903. 


1883-1903. 


Omitting  traces.       ^^^^^S^ 


1883-1903. 


0-5 


.  o 

*3o3 

a  e 

u 

§a 

a 

t,CW 

o 

>< 

- 

<    '    c        - 


.® 

^ 

« 

^ 

« 

c 

s 

s 

a 

o 

o 

> 

03 

a 

'^ 

< 

'K 

< 

October T 

November    0.8 

December 3.3 

January 5.6 

February 7.5 

March 5.8 

April 0.8 


May. 
Season . 


T 
33.8 


T 

9.7 
12.0 
14.5 
33.9 
25.6 

8.0 

T 


1895* 

1S9S 

1897 

1S92 

1899 

1892 

1884 

1893t 


1212 
364 
259 
452 
441 

1000 


61.1  1898-9    215 


0 

0 

0 

0.2 

2 

0 

T 

0.7 

4 

0 

1.6 

5 

0 

4.5 

2.2 

9 

0 

4.1 

10 

0 

10.6 

3.1 

9 

0 

5.8 

13 

0 

7.0 

3.4 

12 

0 

5.6 

14 

0 

15.5 

o  o 

6 

0 

3.8 

8 

0 

12.0 

0.4 

o 

0 

O.S 

3 

0 

8.0 

0 

0 

0 

0.1 

1          0 

T 

12 

26 

o 

22 

40          6 

15.5 

in 

in 

in       in 

1884-5 

1877-8 

1892-31875-6 

1895* 

1898 

1887 

1892 

1899 

1892 

1884 

1893t 

1899 

in 

Feb. 


T  Indicates  a  "  trace ' 

•  Also  1889  and  1890. 


of  snow ;  an  amount  less  than  0.1  inch. 

+  Also  1891. 


16  years,  and  hence  are  fairly  relial)le.     Trace?  of  snow,  or  snow  ''  flur- 
ries," were  included  in  the  calculations. 

DURATION  OF   SNOWFALL. 
(Including  traces.) 


Nov. 

Dec. 

Jan. 

Feb. 

March 

April 

Season 

11 
60 
5.30 

45 

2;» 

5.20 

99 
428 
4.20 

67 
395 
5.50 

34 

169 

5.00 

10 

47 

4.45 

266 

"      duration  in  hours 

Average  duration  (in  hrs.  and  miu.) 

1S^8 
5.00 

Fogs. 

Fogs  in  the  vicinity  of  Baltimore  are  confined  mostly  to  the  fall,  win- 
ter and  early  spring  months.  The  record  contained  in  Table  LXI 
applies  only  to  dense  fogs  surrounding  the  local  station  of  the  U.  S. 
Weather  Bureau  office.  Their  frequency  is  doubtless  greater  as  the 
harbor  or  the  bay  is  approached.  A  fog  has  been  regarded  as  dense 
when  it  obscured  objects  at  a  distance  of  about  1000  feet,  and  it  has 


238 


THE    CLIilATE    OF    BALTI:M01{E 


been  recorded  only  when  it  hung  abont  the  station  for  one  hour  or  more. 
The  table  includes  only  such  fogs  as  have  been  described  and  which 
occurred  since  1891,  the  earlier  records  being  regarded  as  less  reliable. 

TABLE  LXI.— FREQUENCY  OF  DENSE  FOGS.* 


Year. 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Ann'l. 

1891 

3 

i 

3 

1 

1 
1 
5 
2 

1 

i 

4 

23 

1.8 

1 

i 

4 
1 
1 

i 

4 

17 
1.3 

1 

i 

3 
3 

3 

1 

'8 

18 
1.0 

'i 

1 
1 

i 

4 
0.3 

1 
1 

2 
0.2 

1 

1 
0.1 

0 
0 

i 
i 

o 

0.2 

2 
1 

2 

6 
0.4 

i 
1 

3 

"b 

7 

3 

1 

20 
1.6 

2 

"i 

3 
5 

1 

i 

i 

3 
6 
2 

37 
2T1 

6 

"b 

1 
3 

2 

1 
2 

i 

3 
6 

31 
2.4 

13 

1893. 

1893 

0 
9 

1894   

1895 

10 
15 

1896  

13 

1897 

1898 

1899 

1900 

8 
9 

13 
14 

1901 

13 

]<i02 

15 

1903    

19 

Total  (13  yrs.) 

Aver,  per  jear ... 

150 
11.5 

*  Fog  about  station  for  one  hour  or  more,  and  too  dense  to  see  objects  at  1000  feet. 

During  the  13  years  from  1891  to  1903  there  were  150  dense  fogs  re- 
corded, or  about  12  per  year.  The  percentage  of  occurrence  in  the  differ- 
ent months  of  the  year  is  shown  by  the  following  figures : 


Jan.  Feb.  Mar.  Apr.  May  June  July  Aug.  Sept.  Oct.    Nov.  Dec.  Year 


Percentage of  total 
annual  number 


15 


13 


18 


100 


Arranging  the  months  in  the  order  of  frequency  of  occurrence  of  fogs, 
we  have:  December,  ISTovember,  January,  October,  March,  February, 
September,  April,  May  and  August,  June,  July.  These  fogs  are  not  of 
long  duration,  rarely  continuing  any  considerable  portion  of  the  day. 
When  they  do  occur,  however,  they  are  a  serious  menace  to  the  shipping 
interests  in  the  harbor.  A  complete  list  of  the  dates  of  occurrence  of  all 
dense  fogs  about  the  local  station  of  the  U.  S.  Weather  Bureau  is  given 
in  Table  LXII.  The  annual  number  since  1891  has  varied  from  19  in 
the  year  1903  to  none  in  the  year  1892.     None  have  been  recorded  in 


^iIARYLAXD    WEATHER    SERVICF 


239 


TABLE  LXII.-DATES  OF  OCCURRENCE  OF  DENSE  FOGS.* 


1S91 

1894 

1896 

1899 

1901 

1903 

Jan.       1 

Jan. 

11 

Jan.      29 

Jan. 

14 

March 

25 

Jan.       1 

16 

Feb.        1 

24 

May 

24 

27 

11 

24 

4 

Feb. 

18 

June 

14 

28 

Feb.     21 

Feb. 

9 

"            5 

21 

Oct. 

10 

29 

March    9 

Aug. 

30 

29 

March 

4 

" 

31 

Feb.       2 

Nov.     21 

Oct. 

20 

March  26 

6 

Nov. 

1 

4 

"         22 

Nov. 

2 

"       30 

" 

18 

16 

11 

Dec.     "3 

21 

April     6 

Oct. 

11 

22 

28 

"        2'^ 

23 

Nov.     26 

" 

1.5 

Dec. 

1 

March    4 

23 

Dec. 

12 

Dee.       7 

" 

17 

2 

8 

24 

30 

26 

9 

"       11 

25 

189.=) 

Dec. 

~i 

" 

13 
29 

17 

26 

19 

1897 

"       20 

21 
"       24 

1892 

Jan. 

21 

1900 

1902 

March 
Sept. 

5 

Jan.        3 
Feb.        6 

April      8 

Nov.     10 

23 

0 

11 

March  19 

Jan. 

19 

Jan. 

18 

Oct. 

2 

21 

Feb. 

8 

Feb. 

27 

1904 

'' 

April      6 
Nov.       5 

Aug-. 
Oct. 

30 

May 

28 
19 

1893 

26 

Nov. 

6 

21 

23 

Sept. 

3 

Jan.      22 

8 

Dec.       9 

" 

24 

" 

20 

Feb.       7 

Jan.        1 
April    29 

IT 

>' 

35 

Oct. 

19 

JIarch    2 

19 
25 
18 

1S98 

" 

26 

Nov. 

1 
0 

3 

Oct       1'' 

Dec. 

" 

28 

3 

Nov.       2 
Dec.       7 

9 

10 
23 

28 

31 

" 

19 

Jan.       6 

Nov. 

25 

** 

5 

April      1 

" 

28 

11 

Dec. 

19 

" 

14 

9 

12 

20 

" 

15 

June      4 

13 

" 

22 

Dec. 

3 

Sept.    14 

20 
Feb.      10 

16 

Oct.      10 
Nov.       3 

Nov.       5 

4 

Dec.     20 

24 

21 

Dec.     27 

*  Fog  about  station  for  one  hour  or  more,  and  too  dense  to  see  objects  at  1000  feet. 

the  month  of  July  during  this  period  of  13  years,  and  but  one  in  the 
month  of  June,  two  each  in  May  and  August. 


SUXSHINE    AXD    CLOUDINESS. 

Sunshine. 

In  connection  with  a  discussion  of  the  amount  of  sunshine  recorded 
at  Baltimore,  it  is  important  to  know  the  metliod  employed  in  obtaining 
the  record.  The  instrument  in  use  at  the  local  office  of  the  Weather 
Bureau  since  1893  is  of  the  kind  known  as  the  electrical  thermometric 
recorder.  The  essential  parts  of  the  instrument  are  the  two  glass 
bull)S,  one  of  which  is  covered  with  lamp-black.  The  two  Inilbs  are 
joined  by  a  tube,  in  the  middle  portion  of  which  are  the  terminals  of  an 
electric  circuit.     The  direct  rays  of  the  sun  falling  upon  the  Itlack  bulb 


240  THE    CLIIMATE    OF    BALTIMORE 

will  raise  the  temperature  of  the  air  within  to  a  higher  degree  than  that 
within  the  bright  bulb.  This  difference  in  temperature  sends  a  column 
of  mercury  to  the  terminals  in  the  connecting  tube.  When  the  sun 
passes  behind  a  cloud  or  below  the  horizon,  or,  in  other  words,  when  the 
direct  rays  of  the  sun  do  not  fall  upon  the  bulb,  the  temperature  in  both 
is  presumably  the  same,  the  mercury  column  remains  below  the  terminals 
and  the  circuit  remains  open  within  the  instrument.  A  recording  de- 
vice is  placed  in  the  electric  circuit  at  some  convenient  point  in  the  ob- 
serving station.  While  the  sun  shines  upon  the  black  and  bright  bulbs, 
a  characteristic  line  is  drawn  by  a  pen  upon  the  revolving  drum  of  the 
recording  instrument.  While  the  circuit  is  open  a  straight  line  is  pro- 
duced. The  clock  which  forms  part  of  the  recording  device  closes  the 
circuit  every  minute  of  the  day  and  night.  In  this  manner  we  obtain  a 
record  of  sunshine  or  no  sunshine  once  every  minute  between  sunrise 
and  sunset.  At  the  close  of  the  day  we  may  then  add  up  the  number 
of  minutes  of  sunshine.  With  these  figures  and  knowing  the  exact 
number  of  hours  and  minutes  between  sunrise  and  sunset,  we  may  obtain 
the  percentage  of  possible  sunshine  for  each  day. 

The  hourly  records  for  ten  years  show  that  there  is  a  steady  increase 
in  the  amount  of  sunshine  in  all  months  from  sunrise  to  a  maximum 
at  about  noon.  The  maximum  hourly  amount  increases  from  04  per 
cent  in  January  to  81  per  cent  in  September,  and  then  again  decreases 
to  a  minimum  in  January.  The  hourly  distribution  is  shown  in  terms 
of  percentages  of  the  possible  amount  for  each  hour  and  month  of  the 
year  in  Table  LXIII.  The  same  distribution  is  graphically  shown  in 
Fig.  66,  in  which  increase  in  the  intensity  of  shading  rei^resents  an  in- 
crease in  the  amount  of  sunshine.  In  Fig.  67  the  average  increase  from 
hour  to  hour  for  the  entire  year  is  indicated  by  means  of  a  single  curve ; 
this  shows  a  rapid  and  very  uniform  increase  from  sunrise  to  noon,  and  a 
similar  decrease  to  sunset.  This  law  of  variation  is  common  to  all  months 
of  the  year.  The  fact  should  not  be  overlooked  that  the  amount  of  sun- 
shine recorded  as  described  above  is  not  a  complement  of  the  amount  of 
cloudiness.  Sunshine  may  be,  and  frequently  is,  recorded  when  the  sky 
is,  to  a  great  extent,  clouded.     The  instrumental  record  only  indicates 


MARYLAND    WEATHER    SERVICE 


241 


TABLE  LXIII.-AVERAGE  HOFKLY  DURATION  OF  SUNSHINE. 


Hours. 

» 

2 

a, 

>> 

0 

G 

"3 

3 

+3 

a 

0 

> 

0 

§'   = 

•^ 

tn 

i 

< 

S 

»-5 

HS 

< 

E» 

0 

•^ 

~'     "1 

4-  5  a.  m 

29 

40 

44 

9 

5-6    '•     

33 

41 

.S2 

42 

42 

38 

49 

23 

6-7    "     

33 

31 

44 
,54 

38 
.50 

49 
61 

48 
61 

41 

55 

48 
57 

38 
43 

28 

..     33 

7-8    "     ...   

9H 

29    44 

g^-  9    "      

m 

4fi 

.51 

64 

.58 

70 

68 

66 

67 

53 

40 

38     55 

9-10    '•      

+9 

m 

63 

68 

65 

75 

77 

73 

74 

64 

,52 

54     64 

10  IJ    ••     

6.S 

68 
74 

67 
70 

72 

71 
70 

78 

79 

79 
SO 

80 
81 

69 
70 

61 
64 

61  ;  70 

11-  Noon 

65  :  72 

Noon-  1  p.  m 

fi+ 

7"' 

72 

74 

71 

79 

80 

80 

80 

72 

65 

65  1  73 

62 

71 

70 

■j-o 

70 

7-* 

80 

77 

77 

72 

64 

63  ;  71 

2-3    ••      

!SX 

66 

66 

68 

66 

75 

75 

72 

74 

68 

57 

56  ,  67 

3-4    "     

JO 
.ST 

61 

f,0 

63 
53 

62 

,56 

.56 
49 

70 
60 

67 
59 

66 

54 

70 
.59 

60 
48 

43 
34 

41 
31 

59 

4-5    "     

49 

&- 6    "     

m 

43 

39 

43 

m 

46 

47 

:{8 

47 

47 

35 

6-  7    "      

36 

83 

25 

m 

30 

27 

46 

19 

7-8     ■•      

•• 

18 

31 

27 

30 

» 

Mean  dailj-  number  of  hours  of  sunshine. . 

4.9 

6.4 

6.8 

7.9 

7  7 

9.2 

9.2 

8.4 

8.4 

6.7 

5.0 

4.9'  7.1 

possible  numlier 

9.8 

10.7 

12.(1 

13.2 

14.314.E 

14.6 

13.7 

12.511.2 

10.1 

9.512.2 

Percentage  of  possible  numV)er 

.50 

.59 

57 

eo 

.54 

02 

63 

61 

67 

60 

M 

51     58 

Table  LXIII  shows  the  average  duration  of  sunshine  for  each  hour  from 
sunrise  to  sunset,  expressed  in  percentage  of  the  possible  amount  of  sunshine. 
The  values  are  based  on  the  continuous  record  of  a  self-registering  thermo- 
metric  sunshine  recorder  during  the  ten  years  from  1894  to  1903.  The 
average  daily  duration  is  also  given  in  hours  and  tenths  and  in  percentage 
of  the  possible  number  of  hours. 


Fig.  M. — Mean  Hourly  Sunshine. 

The  diagram  show.s  the  mean  hourly  sunshine  during  each  hour  of  the  day  and  mouth 
of  the  year,  expressed  as  a  percentage  of  the  liighest  possible  amount  for  the  season. 
It  is  based  on  the  ten  years'  record  of  a  self-registering  thermometric  sunsliine  recorder. 
The  dotted  lines  S.  R.  and  S.  S.  show  the  time  of  sunrise  and  sunset,  respectively.  The 
heaviest  shading  shows  the  time  of  occurrence  of  the  higliest  percentage  of  sunshine. 
The  curved  lines  mark  intervals  of  10  per  cent  in  the  amount  of  sunshine. 


242 


THE    CLIMATE    OF    BALTIMORE 


U'hetlier  the  face  of  the  sun  is  or  is  not  obscured  at  the  moment  of  record- 
ing.    It  is  only  approximately  an  index  of  cloudiness. 

There  is,  in  all  seasons  of  the  year,  an  almndance  of  sunshine  at  this 
station.  The  amount  varies  considerably  in  different  months,  but  in  all 
months  the  average  is  above  50  per  cent  of  the  possible  amount.  Janu- 
ary and  December  have  the  smallest  amount  in  actual  number  of  hours 


Fig.  67. — Mean  Hourly  Sunshine  for  the  Year. 

The  diagram  is  based  on  the  ten  years"  record  of  a  self-registering  thermometric  sun- 
shine recorder.  The  figures  to  the  right  and  left  of  the  diagram  show  the  amount  of 
sunshine,  expressed  as  percentages  of  the  highest  possible  amount. 


as  well  as  in  percentage  of  the  possible  amount.  The  amount  increases 
from  4.8  hours  in  December  to  a  maximum  of  9.2  hours  in  June,  per 
day.  September,  with  but  8.1  hours  of  sunshine,  has  a  higher  per- 
centage than  June,  the  value  for  the  latter  being  62  per  cent,  and  the 
former  65  per  cent. 

The  average  monthly  and  annual  amounts  of  sunshine  are  indicated  by 
the  following-  frg-ures : 


MARYLAND   WEATHER    SERVICE 


243 


Average  Daily  Sunshine. 


Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Year 

Average  in  hours. .     4.9 
"  percen- 
tage of   possible 
amount 50 

6.4 
69 

6.8 
57 

7.9 
60 

7.7 
54 

9.2 
62 

9.1 
62 

8.6 
63 

8.1 
65 

6.8 
60 

5.5 
61 

4.8 
50 

7.2 
58 

The  sunshine  of  an}^  given  month  may  vary  greatly,  however,  from 
that  indicated  by  the  average  figures  given  above.  In  the  following 
table  the  months  of  the  period  from  1893  to  1903,  during  which  the 
greatest  and  least  amount  of  sunshine  prevailed,  are  indicated,  together 
with  the  monthh'  ranges.  The  years  in  which  these  amounts  were 
recorded  may  be  found  by  consulting  Tables  LXIY  and  LXV. 


TABLE  LXIV.— AVERAGE  NUMBER  OF  HOURS  OF  SUNSHINE. 
(By  months  and  years.) 


Year. 


Jan.  Feb.  Mar.  Apr.   May  June  July  Aug-.  Sept.  Oct.   Nov.  Dec.   Ann'l 


1893 .... 

1894 5.7 

189.5 1  5.2 

1S96 4.0 

1M97 3.7 

1898 5.5 

1.^99 6.4 

1900 i  5.2 

1901 !  4.4 

1902 4.6 

1903 4.6 

Average 4.9 


6.2 

8.4 

4.8 
4.3 
7.1 
7.8 

5.8 

7.0 
6.3 
6.3 


7.9 

8.4 

6.2 
6.0 
6.8 
8.8 
5.7 

6.1 

6.4 
5.9 


6.4  ,    6.8 

I 


9.0 

6.2 

7.1 

8.7 

11.0 


5.5 
7.4 
7.4 

7.9 


4.6 
8.2 
9.1 
5.8 
8.1 

5.4 
8.8 
9.7 

7.7 


11.8 
9.0 


7.3 
12.4 
9.5 


10.0 
9.8 
6.1 

9.2 


12.4 
10.1 

7.1 

6.0 

8.8 

10.3 

9.3 

8.0 
8.9 
10.8 

9.1 


9.0 
9.9 
11.0 

6.5 
7.2 
9.7 

8.7 
6.6 
9.2 

6.7 
9.2 
9.9 

7.3 

8.0 

8.4 
7.6 
6.9 

8.6 


7.7 

7.9 
6.7 
9.1 

8.1 


7.0 
6.5 
8.0 

5.0 

5.8 
7.5 
7.0 
4.8 

8.7 
7.3 
6.1 


5.8 
7.0 
4.5 

2.9 

5.6 
5.9 
10.0 
4.0 

4.9 
4.3 
5.6 

5.5 


6.2 
3.4 

3.5 
5.1 

5.7 
4.8 

4.8 

4.8 
4.6 
5.7 

4.8 


8.2 
8.0 

5.5 
6.2 
8.0 
8.1 
6.8 

fi.8 
6.9 
7.0 


TABLE  LXV.— PERCENTAGE  OF  POSSIBLE  SUNSHINE. 
(By  months  and  years.) 


Year. 


1894. 
1H95. 

1896. 
1H97. 

iMOtl. 

1S99. 
1900. 

1901. 
1902. 
1903. 


Jan.  Feb.  Mar.  Apr.  May  June  July  Aug.  Sept.  Oct.  Nov.  Dec.    Ann'l 


Average 


60169        67       60       6462       62       63       66       60 


64 


17 


244 


THE    CLIMATE    OF    BALTIMORE 


HIGHEST  AND  LOWEST  MEAN  MONTHLY  SUNSHINE. 
(In  percentage  of  the  possible  amount.) 


p 

ti 

C 

^ 

e 

c 

>, 

to 

4a 

+5 

> 

d 

§ 

» 

as 

P. 

cS 

a 

3 

P 

u 

o 

» 

1.® 

>-> 

^ 

S 

<! 

3 

1-5 

i-s 

< 

m 

O 

!< 

fi 

rH 

Highest  mean.. 

64 

79 

74 

84 

68 

84 

85 

81 

80 

78 

70 

66 

67 

Lowest  mean.. 

38 

40 

48 

41 

32 

41 

41 

49 

52 

43 

29 

36 

45 

Range 

26 

39 

26 

43 

36 

43 

44 

32 

38 

35 

41 

30 

22 

The  months  of  least  sunshine  show  an  average  of  over  40  per  cent, 

only  the  winter  months  and  the  month  of  May  having  at  any  time  during 
the  eleven  years  fallen  below  this  value.  The  monthly  range  varies 
from  26  per  cent  in  January  and  March  to  44  per  cent  in  the  month  of 
July.  The  average  for  the  entire  year  has  varied  from  67  per  cent  in 
1894  to  45  per  cent  in  1896,  a  range  of  22  per  cent. 


TABLE  LXVL-SUNSHINE  PHASES. 
(Local  time). 

U 

C 

1st  Mean. 

Maximum. 

2d  Mean. 

43 

Time. 
Hours 
ending 
a.  m. 

Value 

Time. 
Hours 
ending 
p.  m. 

Value 

Time. 
Hours 
ending 

p.  m. 

Value 

in 
hours 

O 

s 

January  

February 

March 

7.17 
6.. 52 
6.11 
5.25 
4.47 
4.33 
4.45 
5.13 
5.41 
6.10 
6.43 
7.12 

5.54 

10.10 
10.00 
9.30 
8.40 
8.30 
8.10 
8.20 
8.30 
9.00 
9.40 
10.00 
9.50 

9.15 

50 
59 
57 
60 
54 
62 
63 
61 
67 
60 
50 
51 

.58 

1.00 
12.20 

1.10 
12.30 
12.50 

1.10 
12.20 
12.20 
12.20 
12.50 

1.00 

1.10 

12.45 

64 
74 

72 
74 
72 
79 
80 
80 
81 
72 
65 
65 

73 

4.00 
4.10 
4.30 
4.20 
4.20 
4.. 50 
4.30 
4.30 
4.20 
4.00 
3.30 
3.20 

4.10 

4.9 
6.4 
6.8 
7.9 
7.7 
9.2 
9.2 
8.4 
8.4 
6.7 
5.0 
4.9 

7.1 

5.02 
5.37 
6.07 
6.37 

May 

7.05 
7.26 

July 

7.25 
6.. 55 

September 

October 

6.09 
5.22 
4.46 

December 

Year 

4.39 
6.06 

Table  LXVI  shows  the  time  of  day  when  the  maximum  amount  of 
cloudiness  is  most  likely  to  occur,  and  the  time  which  most  nearly  represents 
the  time  of  occurrence  of  the  average  daily  cloudiness  in  the  morning  and 
afternoon. 

Sunshine  Phases. 

Table  LXVI  indicates  the  hour  of  day  during  which  the  maximum 

amount  of  sunshine  has  occurred   most  frequently  in  the  past  eleven 

years;  also  the  hours  of  the  morning  and  afternoon  during  which  the 

sunshine  is  most  likely  to  be  equivalent  to  the  average  amount  for  the 

day.     These   facts   are   of   importance   in   selecting   the   best  hours   for 

observino^  and  recording  sunshine. 


MARYLAND   WEATHER   SERVICE 


245 


Cloudixess. 

Recording  the  amount  of  cloudiness  has  always  been  an  important 
feature  of  a  regular  observation  in  the  work  of  the  U.  S.  Weather  Bureau. 
A  careful  record  of  the  extent  of  cloudiness  has  been  maintained  at  the 
local  station  of  the  Bureau  since  the  establishment  of  the  Service  in 
Januar}',  1871.  At  the  present  time,  two  direct  observations  per  day 
are  made,  one  at  8  a.  m.  and  the  other  at  8  p.  m.  At  various  times 
since  1871  the  following  constituted  the  recjular  hours  of  observation : 


1    1 

1    1 

I    1 

1  1 

1    1 

1  1 

1  1 

1      1 

^^P 

^^H 

^^^ 

J 

r 

^^!S^ 

^^^ 

B 

B 

Fig.  68. — Average  Hourly  Cloudiness. 

The  diagram  Is  based  on  a  record  of  five  years  of  direct  observations  at  12  stated 
hours  of  the  day  from  7  a.  m.  to  11.30  p.  m.  The  cloudiness  is  rated  on  a  scale  from 
0  to  10,  the  former  figure  representing  a  clear  sky  and  the  latter  an  overcast  sicy.  The 
dotted  line  is  based  on  interpolated  values  from  midnight  to  7  a.  m.,  no  direct  observa- 
tions being  available  for  the  period. 

7  a.  m.,  8  a.  m.,  11  a.  m.,  noon,  2  p.  m.,  3  p.  m.,  4.30  p.  m.,  7  p.  m., 

8  p.  m.,  9  p.  m.,  10  p.  m.  and  11  p.  m.  The  amount  of  cloudiness  is 
noted  in  terms  of  the  number  of  tenths  of  the  sky  covered  at  the  time 
of  observation.  In  order  to  arrive  at  the  law  of  increase  and  decrease 
in  the  diurnal  amount  of  cloudiness,  the  average  extent  of  cloudiness 
was  determined  for  a  period  of  five  years  at  each  of  the  12  hours  of 
observation  mentioned  above.  These  twelve  periods  of  the  day  afford 
ample  material  for  accurately  noting  the  daily  march  of  cloudiness 
between  the  hours  of  7  a.  m.  and  11  p.  m.,  but  leave  a  serious  gap 


246  THE    CLIMATE    OF    BALTI:M0RE 

between  midnight  and  early  morning  which  coukl  onl_v  be  l)ridged  over 
by  interpolating  the  most  probable  values. 

The  following  figures  show  the  average  values  for  the  extent  of  cloud- 
iness at  the  stated  hours  of  the  day: 

AVERAGE   CLOUDINESS. 

(On  a  scale  of  one  to  ten.) 

Hours  of  Observation.  For  the  Year. 

7.00  a.  m 5.3  tenths  of  sky  covered. 

8.00  a.  m 5.1 

11.00  a.  m 5.7 

Noon      5.9       " 

2.00  p.  m 6.0 

3.00  p.  m 5.8 

4.30  p.  m 5.8 

7.00  p.  m 4.8 

8.00  p.  m 4.4 

9.00  p.  m 4.4 

10.00  p.  m 4.3 

11.00  p.  m 4.2 

These  annual  average  values  have  been  graphically  presented  in  Fig. 
68.  The  dotted  contour  line  from  midnight  to  7  a.  m.  indicates  that 
the  curve  is  based  on  interpolated  values.  The  form  of  the  curve  shows 
a  steady  increase  in  cloudiness  from  early  morning  to  a  maximum  at 
2  p.  m.,  and  then  a  somewhat  more  rapid  decrease  to  midnight,  with  a 
probable  minimum  sometime  in  the  early  morning  hours.  The  average 
daily  cloudiness  based  upon  the  8  a.  m.  and  8  p.  m.  observations  is 
somewhat  too  low.  Any  of  the  series  of  three  daily  observations  em- 
ployed in  past  years  by  the  U.  S.  Weather  Bureau,  the  Smithsonian 
Institution  or  the  Army  Medical  Department,  will  yield  a  daily  average 
very  closely  agreeing  with  the  daily  mean  based  on  the  twelve  daily  obser- 
vations distributed  as  noted  in  the  preceding  paragraph. 

Clear,  Partly  Cloudy  ak'd  Cloudy  Days. 

It  has  been  the  custom  for  many  3^ears  to  designate  the  character  of 
the  day  as  clear,  partly  cloudy  or  fair,  and  cloudy,  the  classification 
being  based  upon  the  average  amount  of  cloudiness  at  two  or  more 
stated  hours  of  the  day,  or  upon  prevailing  conditions  for  the  day.  The 
sky  is  designated  as  clear  when  it  is  entirely  free  from  clouds,  or  when 
less  than  one-third  is  covered  by  clouds;   it  is  regarded  as  fair  or  parUij 


MARYLAND    WEATHER    SERVICE 


241 


cloudy  when  covered  to  the  extent  of  four  to  seven  tenths;  and  cloudy 
when  it  is  from  eight  to  ten  tenths  overcast.  There  is  no  instrumental 
method  for  measuring  the  exact  amount  of  cloudiness;  hence  the  classi- 
fication must  be  left  to  the  judgment  of  the  individual  observer.  How- 
ever, it  is  a  convenient  method  of  designating  the  character  of  the  day 
as  regards  the  extent  of  sky  covered  by  clouds  and  is  a  fair  index  of  the 
amount  of  sunshine  received  at  the  observing  station.     The  frequency 


Fig.  69. — Relative  Frequency  of  Clear,  Partly  Cloudy  and  Cloudy  Days. 

of  occurrence  of  days  of  each  of  the  classes  at  a  given  locality  is  a  matter 
of  the  highest  importance  to  health  and  personal  comfort,  and  a  vital 
factor  in  plant  growth. 

All  days  since  18T1  have  been  grouped  into  the  three  classes  described 
above.  The  variation  in  tlie  frequency  of  occurrence  of  clear,  partly 
cloudy  and  cloudy  days  from  month  to  month  and  from  year  to  year  is 
shown  in  Tables  LXVII,  LXVIII  and  LXIX,  and  in  Fig.  69.  The 
mean  monthly  frequency  of  each  class  is  shown  in  the  following  table : 

MKAN   MONTHLY   CLOUDINESS. 
(1871-1902.) 


1 

1 

.Tan. 

Feb.  Mar. 

April 

May 

June  July 

Aug. 

Sept. 

Oct. 

Nov.  1  Dec. 

Year 

Clear  days 

Partly  cloudy 
days 

Cloudy  (lavs  — 

8.3 
12.1 
10..; 

8.4      8.8 

10.9     11.5 

H.!t     10.7 

9.2 
11.8 
9.1 

9.6 
11.6 
9.9 

9.0    10.0 
14.0    13..3 
7.3  1    7.4 

10.7 
12.9 
7.4 

11.9 
10.5 
7.5 

12.4 
10.3 

8.T 

10.2      9.7 
10.2  1  11.5 
9.6       9.4 

118.1 
140.6 
lOfi.ii 

248 


THE    CLIMATE    OF    BALTIMORE 


Frequency  of  Clear  Days. 

The  variation  in  the  number  of  clear  days  from  month  to  month  and 
year  to  year  is  shown  in  Table  LXVII.     Variations  in  the  annual  fre- 


TABLE  LXVII.-NUMBER  OF  CLEAR  DAYS. 
(Less  than  4  tenths  of  sky  covered.) 


Year. 


1871 

1872 

1873 

1874 

1875 

1876 

1877 

1878  

1879  

1880 

1881 

1883 

1883 . 

1884 

1885 

1886 

1887 

1888 , 

1889 

1890 

1891 

1893 

1893 

1894 

1895 

1896 

1897 

1898 

1899 

1900 

1901 

1903 

1903 

Average 

1871-1880 

1881-1S90 

1831-190 J 

1871-1903  .... 


Jan.  Feb.  Mar.  Apr.   May  June  July  Aug.  Sept.  Oct.  Nov.  Dec 


7.9 
6.8 
10.3 

8.3 


7.9 
7.4 
9.1 

8.4 


9.1 
7.3 
9.8 

8.8 


13 


7.9 
8.8 
11.4 


11.1 

7.9 
9.9 

9.5 


7.9 
8.3 
10.7 

9.0 


8.8 
10.3 
11.7 

10.0 


9.3 
10.6 
13.5 

10.7 


11.0 


16 


11.8 


9.3  10.3 
18.0  !  14.5 


9.4 
10.5 
10.9 


11.9  12.4  10.3 


10 


7.8 
9.5 
11.5 


Ann'l 


110 
133 
113 
115 
99 

86 
113 

lis 

133 
10] 

88 
93 
135 
133 

84 

111 

104 
114 
100 
137 

147 
143 
155 
166 
154 

97 
126 
141 
147 
107 

122 
107 
113 


109.9 
106.9 
138.3 

118.1 


quency  are  also  graphically  shown  in  Fig.  69  in  connection  with  the 
partly  cloudy  and  cloudy  days.  During  the  course  of  a  year  we  may 
count  on  about  118  clear  days,  or  days  with  a  cloud  covering  of  three- 
tenths  or  less.  The  annual  frequency  has  varied  greatly  from  1871  to 
1903.  In  1885  but  84  were  recorded,  while  in  1894  there  were  166,  or 
about  double  the  number.     The  table  shows  a  rather  remarkable  increase 


MARYLAND   WEATHER   SERVICE 


249 


in  the  ten-year  averac^e  for  1891  to  1900  (138)  over  that  of  the  decades 
from  1871-1880  (110)  and  1881-1890  (107),  a  variation  which  is  diffi- 
cult to  account  for,  considering  the  close  approximation  of  the  values 
for  the  two  preceding  ten-year  periods. 


TABLE  LXVIII.— NUMBER  OF  PARTLY  CLOUDY  DAYS. 
(From  4  to  7  tenths  of  sky  covered.) 


Year. 


1871. 
1872. 
1873. 
1874. 

1875. 


1876. 
1877. 
1878. 
1879. 
1880. 

1881. 
1883. 
1883. 
1884. 
1885. 

1886. 
1S87. 
1888. 
1889. 
1890. 

1891. 
1893. 
1893. 
1894. 
1895. 

1896. 
1897. 
1898. 
1899. 
1900. 

1901. 
1903. 
1903. 


Average. 


Jan. 


1871-1880 13..") 

1881-1890 13.4 

1891-1900 10.8 

1871-1903 1  12.1 


Feb, 


Mar. 


12.3    11.4 
11.8    13.3 


9.3 
10.9 


9.8 
11.5 


Apr. 


12.1 
13.5 
10.3 

11.8 


May 


12.0 
12.4 
10.8 


11.6 


June 


14.7 
14.9 
12.6 

14.0 


July 


14.0 
13.3 
12.6 

13.3 


Aug. 


11.5 
14.1 
13.6 

12.9 


Sept, 


10 
10 
14 
8 
13 

9 

10 
10 
13 
10 

14 
13 
13 
11 
16 

19 

15 

8 

8 

14 

11 
6 
9 

7 
8 

10 
6 
4 
8 

10 

12 

9 
10 


10.7 
13.0 

7.9 

10.6 


Oct. 


11.9 

11.3 

7.9 


Nov. 


11.4 

9.8 
9.9 


10.2    10.2 


Dec, 


Li 


13.0 
13.5 
10.0 


Ann'l 


148 
160 
157 
143 
102 

164 
130 
131 
142 
148 

164 
173 
154 
140 
183 

157 
160 
131 
1.30 
131 

112 
140 
112 
118 
141 

136 
122 
116 
111 

137 

119 
126 
114 


US.  5 
1.52.2 
124.5 


140.5 


September  and  October  have  the  highest  percentage  of  clear  days, 
followed  closely  by  August,  November  and  July.  The  minimum  fre- 
quency occurs  in  January.  There  is  an  almost  uniform  increase  in  the 
number  from  January  to  a  ma.ximum  in  October,  followed  by  a  steady 
decrease   to   a    inininuiiii    in    January.     The   only    interruption    in    the 


250 


THE    CLIMATE    OF    BALTIMORE 


regularity  of  the  increase  is  a  slight  falling  off  in  frequency  in  the 
month  of  June.  In  the  months  of  September  and  October  the  number 
of  clear  days  has  occasionally  reached  20  out  of  the  total  of  30  or  31 
days;  the  number  sometimes  falls  as  low  as  6  or  7;  the  average  fre- 
quency for  September  is  11.9,  and  for  October  12.4. 


TABLE  LXIX.-NUMBEK  OF  CLOUDY  DAYS. 

(Over  7  tenths  of  sky  covered.) 


Year. 

1871 

1873 

1873 

1874 

1875 

1876 

1877 

1878  

1879 

1880 

1881 

1882 

1883 

1884 

1885 

1886 

1887 

1888 

1889 

1890 

1891 

1892 

1893 

1894 

1895 

1896. 

1897  

1898 

1899 

1900 

1901 

1902 

1903  

Average. 

1871-1880 

1881-1890 

1891-1900 

1871-1903 


Jan. 
13 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

6 

10 

6 

8 

6 

7 

13 

3 

7 

11 

6 

5 

9 

11 

4 

9 

6 

8 

11 

11 

8 

6 

13 

10 

10 

7 

16 

10 

5 

10 

6 

13 

3 

11 

9 

5 

8 

10 

17 

5 

n 

10 

10 

10 

9 

6 

8 

11 

7 

1.T 

13 

6 

11 

10 

10 

10 

13 

9 

11 

13 

8 

5 

10 

10 

14 

8 

13 

7 

6 

6 

7 

12 

5 

16 

6 

5 

5 

11 

16 

16 

8 

14 

10 

8 

14 

5 

3 

13 

7 

11 

10 

15 

1 

5 

8 

16 

5 

8 

8 

6 

5 

3 

4 

10 

12 

12 

9 

7 

7 

in 

6 

9 

6 

8 

8 

17 

5 

6 

8 

11 

4 

9 

8 

13 

8 

9 

6 

/** 

13 

13 

7 

6 

7 

9 

8 

13 

13 

10 

9 

16 

5 

7 

7 

13 

13 

10 

9 

8 

13 

14 

6 

13 

12 

10 

6 

11 

4 

8 

7 

8 

10 

15 

6 

13 

8 

10 

11 

11 

13 

11 

11 

6 

3 

3 

4 

8 

9 

9 

10 

9 

10 

4 

6 

6 

8 

7 

8 

11 

o 

3 

3 

8 

2 

6 

8 

4 

6 

8 

2 

13 

11 

12 

9 

14 

13 

13 

5 

13 

16 

13 

7 

11 

7 

10 

7 

11 

8 

14 

9 

13 

2 

7 

6 

11 

U 

14 

5 

10 

6 

6 

10 

11 

11 

15 

10 

13 

10 

3 

5 

13 

4 

9 

18 

16 

6 

10 

9 

14 

13 

10 

13 

13 

10 

13 

6 

11 

12 

13 

14 

10 

16 

6 

16 

9.6 

8.1 

10.6 

10.0 

7.9 

7.4 

8.3 

10.3 

11.8 

9.0 

10.5 

7.7 

10.7 

6.8 

7.5 

6.3 

9.9 

9.8 

11.4 

8.3 

10.3 

7.7 

5.7 

6.9 

10.6 

8.9 

10.7 

9.1 

9.9 

7.3 

7.4 

7.4 

Sept, 


8.3 
7.7 
6.1 

7.5 


Oct. 


15 


7.3 
10.5 


8.7 


Nov. 

Dec. 

15 

8 

6 

8 

4 

8 

6 

12 

10 

5 

13 

10 

8 

11 

11 

12 

4 

18 

15 

10 

11 

9 

7 

5 

8 

5 

9 

13 

13 

8 

8 

11 

4 

11 

16 

7 

15 

11 

6 

10 

6 

8 

7 

10 

9 

6 

fi 

6 

10 

14 

11 

13 

10 

10 

13 

10 

n 

7 

10 

11 

13 

13 

13 

13 

8 

10 

9.2 

10.3 

9.7 

8.0 

9.3 

9.6 

9.6 

9.4 

Ann'l 


107 

83 
96 
107 
104 

116 
133 
116 
101 

io- 
ns 

100 

86 

103 


101 
121 
135 

107 

106 
83 
98 
81 
70 

133 
117 
108 
107 
121 

124 
132 
138 


106.9 
106.1 
103.4 

106.6 


Frequency  of   Partly   Cloudy  Days. 
The  details  concerning  the  monthly  and  annual  distribution  of  partly 
cloudy  days  may  be  learned  by  consulting  Table  LXVIII.     The  average 
annual  frequency  is  140  days,  with  a  maximum  occurrence  of  182  in 


MARYLAND    WEATHER    SERVICE  251 

1885  and  a  minimum  of  111  in  1899.  The  partly  cloudy  days  are  most 
frequent  in  June  and  least  frequent  in  October  and  ^NTovember.  There 
is  a  fairly  uniform  distribution  throughout  the  year,  the  monthly  aver- 
ages varying  only  between  a  minimum  of  10.2  and  a  maximum  of  14.0, 
as  shown  in  Table  LXYIII.  The  annual  variations  are  shown  graphic- 
ally in  Fig.  69. 

Cloudy  Days. 

.The  frequency  of  occurrence  of  cloudy  days  during  each  month  and 
year  since  1871  is  shown  in  Table  LXIX.  The  average  annual  number 
for  the  entire  period  of  33  years  has  been  about  107,  with  a  maximum 
frequency  of  138  in  1903  and  a  minimum  of  70  in  1895.  Cloudy  days 
have  been  most  frequent  in  the  months  of  March  (10.7)  and  January 
(10.6)  and  least  frequent  in  the  month  of  June  (7.3).  The  average 
annual  variation  is  shown  graphically  in  Fig.  69. 

THE    WINDS. 

INTRODUCTION. 

A  Kobinson  anemometer  with  a  continuous  recording  attachment  has 
been  in  operation  since  the  establishment  of  the  oflQce  of  the  U.  S. 
Weather  Bureau  in  January,  1871.  Hence  we  have  an  excellent  and 
complete  record  of  the  hourly  changes  in  the  velocity  of  the  wind  for  a 
period  of  34  years.  While  it  is  a  matter  of  great  importance  to  have  a 
permanent  observatory  for  meteorological  observations,  it  is  a  difficult 
problem  for  the  National  Weather  Service  to  secure  such  permanence  in 
large  and  rapidly  growing  cities  where  changes  in  neighboring  buildings 
so  alter  the  conditions  of  exposure  of  instruments  as  to  make  a  change 
in  the  location  of  the  observatory  a  necessity.  Since  1871  the  successive 
changes  in  the  elevation  of  the  anemometer  were  as  follows : 

CHANGES  IN  THE  ELEyATION  OW  THE  ANEMOMETER. 

Above  Ground.  Above  Sea-leveL 

1873  to  Oct.  12,  1878  75  feet.  90  feet. 

1878  to  Jan.  1,  1889  SO  "  100 

1889  to  May.  1891  100  "  120 

1891  to  Sept.  7.  lSn.->  100  "  208 

1895  to  Auk.  1.  189G  136  "  173 

1896  to  Apr.  30,  1902  82  "  185 

1902  to  Dec.  1903  117  "  220 


'i.yi  THE    CLIMATE    OF    BALTIMORE 

The  exposure  of  the  anemometer  was  very  satisfactory  during  the 
entire  period,  excepting  from  1896  to  1902,  when  neighboring  buildings 
obstructed  the  free  movement  of  the  atmosphere  over  the  station.  The 
elevation  of  the  anemometer  above  sea  level  was  approximately  the  same 
from  1871  to  1889,  namely,  between  90  feet  and  100  feet;  from  1891 
to  1904,  with  the  exception  of  September,  1895,  to  July,  1896,  the  sea- 
level  elevation  was  increased  by  approximately  100  feet.  Changes  in 
elevation  above  sea  level  affect  the  velocity  of  movement  of  the  atmosphere 
no  less  than  changes  in  elevation  above  ground.  The  abrupt  increase  in 
the  velocity  shown  from  1890  to  1891  is  doubtless  due  to  the  change  in  the 
sea-level  elevation  of  the  anemometer. 

Since  1893  a  continuous  record  of  wind  direction  has  been  maintained 
without  interruption  excepting  for  a  few  hours  at  a  time  when  difficulty 
was  experienced  with  the  recording  instrument.  The  hourly  changes  in 
wind  direction  discussed  in  the  following  pages  are  based  upon  the  ten- 
years'  record  from  1893  to  1902,  unless  otherwise  stated. 

Average  Hourly  Wind  Movement. 

The  recorded  hourly  velocities  for  the  twenty-year  period  from  1881 
to  1900  have  been  reduced  to  average  hourly  values  in  order  to  determine 
the  periodic  variations  in  velocity  during  the  day.  The  results  are 
shown  in  Table  LXX,  and  graphically  in  Figs.  70  and  71.  In  Fig.  70 
the  hourly  changes  in  velocity  are  given  for  the  months  of  January, 
April,  July  and  October,  and  the  average  for  the  entire  year.  The  curves 
for  all  months  are  similar  in  form.  There  is  a  minimum  velocity  in  all 
months  just  before  sunrise.  The  velocity  rises  rapidly  to  a  maximum 
between  two  or  three  in  the  afternoon,  which  it  maintains  approximately 
for  two  or  three  hours,  then  decreases  rapidly  to  8  p.  m.  or  9  p.  m.,  and 
more  slowly  to  the  minimum  for  the  day  just  before  sunrise.  The  same 
hourly  variation  is  shown  for  all  months  of  the  year  in  another  manner 
in  Fig.  71.  The  influence  of  the  diurnal  variations  in  temperature  upon 
the  coincident  variation  in  wind  velocity  is  strikingly  exhibited  in  the 
table  and  diagrams;  the  increase  in  velocity  accompanies  the  increase  in 
temperature   throughout  the   course.     The   time   of   maximum    rate   of 


MARYLAND    WEATHER    SERVICE 


increase  and  decrease  in  velocity  is  coincident  with  the  time  of  maximum 
rate  of  change  in  temperature,  the  most  rapid  increase  occurring  between 


1234.56789    10  II    tn 


ftlttS 

- 

/ 

\ 

/« 

/ 

\ 

\ 

^ 

I 

1 

; 

1 

__^_ 

J 

Ju     Fu    Ken    lor     May   June   Juljf     Au^.  Stu    Oct     No     Dec 

Annual  Vabiations  or  Wind  Velocity 


Fig.  70. — Hourly  and  Annual  Variations  of  Wind  Velocity. 

Expressed  In  miles  and  tenths  of  miles  per  hour  for  the  months  of  January,   April, 
.Tuly  and  October,  and  for  the  entire  year. 

8  a.  m.  and  10  a.  m..  and  the  most  rapid  decrease  between  6  p.  m.  and 
8  p.  m. 

In  the  annual  fluctuation  in  velocity,  however,  a  similar  relationship 
does  not  exist.     On  tlie  contrary,  there  is  almost  a  direct  inversion  of 


354 


THE    CLIMATE    OF    BALTIMORE 


TABLE  LXX.-AVERAGE  HOURLY  WIXD  MOVEMEXT. 
(In  miles  and  tenths.] 


4 

p 

'u 

< 

^ 

S 

2 

3 

>. 

■^ 

bib 

3 
<5 

a. 

X 

O 

o 
2; 

® 

3 

<5 

Midn't  to  1  a.  m.  . . 

5.5 

5.9 

6.0 

5.3 

4.5 

4.2 

4.0 

3.5 

4.0 

4.6 

4.8 

5.1 

4.8 

•J     •■     

5.0 

5.8 

5.9 

5.2 

5.0 

4.1 

4.0 

3.5 

3.9 

4.5 

4.8 

5.0 

4.8 

3     "    .... 

5.3 

5.7 

5.8 

5.1 

4.3 

3.6 

3.9 

3.5 

3.9 

4.4 

4.8 

5.0 

4.6 

i     "    .... 

5.3 

5.8 

5.8 

5.0 

4.1 

3.8 

3.8 

3.6 

3.9 

4.6 

4.9 

5.0 

4.6 

5     "     .... 

5.2 

5.8 

6.0 

4.8 

4.3 

3.9 

3.7 

3.7 

3.9 

4.6 

4.8 

4.9 

4.6 

6      "     .... 

5.1 

5.b 

5.9 

4.8 

4.3 

4.0 

3.8 

3.7 

4.0 

4.6 

4.7 

4.9 

4.6 

7      "     .... 

5.4 

5.9 

6.0 

5.3 

5.0 

4.8 

4.3 

3.9 

4.3 

4.7 

4.8 

5.0 

4.9 

8      "     .    . 

5.6 

6.2 

7.0 

6.4 

5.7 

5.7 

5.3 

4.7 

4.9 

5.4 

5.2 

5.2 

.'1.6 

9      "     .... 

6.1 

7.0 

■    8.5 

7.6 

6.6 

6.3 

6.0 

5.5 

5.8 

6.2 

6.0 

5.7 

6.4 

10      "     .... 

6.9 

7.8 

9.0 

8.2 

7.1 

6.9 

6.5 

6.0 

6.5 

7.3 

7.1 

6.6 

7.3 

11      "     .... 

7.5 

8.3 

9.3 

8.7 

7.7 

7.4 

6.8 

6.5 

6.8 

7.7 

7.7 

7.3 

7.6 

Noon 

7.8 

8.7 

9.6 

9.0 

8.2 

7.7 

7.2 

6.8 

7.2 

7.9 

8.1 

7.7 

8.0 

1  p.  m  — 

8.1 

9.1 

9.9 

9.4 

8.6 

8.1 

7.8 

7.1 

7.5 

8.2 

8.4 

7.S 

8.3 

O          " 

8.3 

9.1 

9.9 

9.7 

8.8 

8.2 

8.0 

7.3 

7.6 

8.2 

8.4 

8.0 

8.4 

3     "     ..  . 

8.2 

9.1 

10.0 

9.7 

8.9 

8.3 

8.2 

7.5 

7.6 

8.2 

8.3 

7.9 

8..'. 

4      "     .... 

7.7 

8.8 

9.9 

9.5 

8.6 

8.3 

8.0 

7.6 

7.6 

8.0 

7.9 

V.5 

8.3 

5      "     .... 

7.1 

8.3 

9.6 

9.1 

8.4 

8.1 

7.6 

7.2 

7.0 

7.1 

6.9 

6.6 

V.8 

6      '•     .... 

6.2 

7.3 

8.4 

8.3 

7.6 

7.3 

7.1 

6.4 

6.0 

5.7 

6.9 

6.8 

6.8 

7     "    .... 

5.8 

6.6 

7.3 

6.8 

6.4 

6.1 

6.0 

5.2 

4.8 

4.9 

5.4 

6.6 

5.9 

8      "     . 

5.6 

6.3 

6.6 

5.8 

6.5 

5.1 

4.7 

4.2 

4.4 

4.9 

6.3 

5.4 

5.3 

9     " 

5.5 

6.0 

6.5 

5.7 

5.2 

4.6 

4.4 

3.9 

4.4 

4.8 

5.1 

5.3 

5. J 

10     "     .... 

5.5 

5.9 

6.3 

5.5 

4.8 

4.4 

4.2 

3.7 

4.3 

4.7 

5.0 

5.2 

5.0 

11      "     ... 

5.4 

6.0 

6.3 

5.4 

4.8 

4.3 

4.0 

3.7 

4.3 

4.7 

6.0 

5.2 

4.9 

Midn't.  ... 

5.5 

5.9 

6.1 

5.3 

4.8 

4.2 

4.0 

3.6 

4.1 

4.6 

4.9 

5.1 

4.8 

Means 

6.3 

7.0 

7.6 

6.9 

6.2 

5.8 

5.6 

5.1 

5.4 

5.8 

6.0 

5.9 

6.1 

Table  LXX  is  based  on  the  continuous  record  of  a  self-registering  anemom- 
eter during  the  20  years  from  1881  to  1900. 


t  2         3         4         5         6  7  8         9 


Fig.  71. — Average  Hourly  Variations  in  Wind  Velocity. 

The  heaviest  shading  shows  the  time  of  occurrence  of  the  highest  average  wind  veloci- 
ties for  the  day.  The  curved  lines  mark  intervals  of  half  a  mile  in  the  average  velocity. 
The  dotted  lines  marked  S.R.  and  S.S.  show  the  time  of  sunrise  and  sunset,  respectively. 
The  diagram  is  based  on  hourly  values  for  a  period  of  20  years. 


MARYLAXD    WEATHER    SERVICE 


255 


the  relation  existing  between  temperature  and  wind  velocitA'.  The  light- 
est winds  occur  in  the  months  of  greatest  heat,  while  the  highest  veloci- 
ties occur  in  March,  with  a  slight  secondary  increase  in  October  and 
November  (see  Fig.  71).  The  annual  fluctuations  are  due  to  the  varia- 
tions in  cyclonic  activity  at  different  seasons  of  the  3'ear,  The  highest 
average  hourly  wind  velocities  occur  between  2  p.  m.  and  3  p.  m.  in  the 
month  of  March,  when  they  attain  an  average  velocity  of  10  miles  per 
hour.  The  lowest  velocities  occur  in  the  early  morning  hours  of  June, 
July  and  August,  when  the  average  falls  to  about  3.5  miles  per 
hour.  This  law  of  increase  and  decrease  is  remarkably  constant  through- 
out the  year  and  is  recognizable  at  any  time  when  not  interrupted  by 
the  presence  of  a  well-developed  cyclonic  or  anti-cyclonic  disturbance. 


Average  Daily,  and  Total  Monthly  Wixd  Movement. 

In  Table  LXXI  the  total  monthly  wind  movement  for  each  month  of 
the  year  from  1873  to  1903  is  shown,  together  with  the  average  daily 
movement  for  each  year  during  the  same  period.  As  the  elevation  of 
the  anemometer  was  changed  several  times  during  this  period,  it  is  essen- 
tial to  bear  in  mind  the  fact  in  discussing  the  variations  in  wind  veloci- 
ties as  shown  in  Table  LXXI.  Xo  attempt  has  been  made  to  reduce  the 
records  to  a  single  elevation;  the  changes  in  elevation  are  distinctly 
traceable  in  the  monthly  and  daily  values  for  the  wind  movement.  In- 
ferences as  to  fluctuations  in  the  annual  velocity  should  be  made  with 
caution.  The  average  daily  wind  movement  is  approximately  145  miles 
for  the  entire  year.  The  velocity  varies  from  a  minimum  of  122  miles 
in  August  to  a  maximum  of  175  miles  per  day  in  March.  The  following 
figures  represent  the  average  daily  wind  movement  for  each  month,  as 
derived  from  hourly  observations  from  1873  to  1902,  a  period  of  30 
years : 

AVERAGE   DAILY   WIND   MOVEMENT. 


Jan. 

Feb. 

Mar. 

175 

Apr. 

May  June  July  Aug. 
149       142       134       122 

Sept. 
129 

Oct. 
137 

Nov. 
143 

Dec. 

Year 

Miles 

146 

162 

160 

142  1    14S 

256 


THE    CLIMATE    OF   BALTIMORE 


TABLE    LXXL— TOTAL    MONTHLY  AND    AVERAGE    DAILY    WIND    MOVEMENT. 


Year. 

© 

PR 

1 

< 

e 
>-> 

3 

1-5 

bo 

3 
< 

® 

0 
0 

0 

0 

Oi 

Q 

c  5 

0  0 

S  i 

m 

1873 

3665 

4848 
3571 

4510 

3764 
5335 
5.33t> 

3R.3S 
4180 
4136 

4636 
3671 
4021 

633S 
5S11 
3977 

5766 
4854 
5124 
4158 
6295 

6519 

4466 
4753 
4615 
4876 

5847 
5893 
5495 
5.336 
4964 

6033 
7326 
6904 

5872 
7070 

8038 
4.33S 
4226 
5040 
4340 

E007 
4594 
4938 

5330 
5.514 
5543 

5435 
175 

513S 
5565 
4891 

4718 
4065 
4870 
5149 
6398 

5045 
4423 
4583 
5139 
4.3a3 

4085 
4963 
4736 
5455 
3997 

44.30 
64.33 

6604 
6351 

6308 

6056 
3974 
4S.39 
4077 
3574 

5-387 
4963 
6449 

4936 
4815 
5303 

4983 
166 

4389 
4670 
4990 

4495 
4016 
4344 
4350 
4764 

3841 
4310 
4993 

4483 
3917 

4335 

3783 
4041 
419.S 
3970 

4838 
5987 
65S8 
5S80 
5435 

5669 
4196 
4235 
36.50 
4101 

4359 
5933 
6336 

4397 
4444 
5004 

4616 
149 

4136 

3966  3.f;73 

3680 
4032 
3353 

4764 
3775 
3937 
3610 
3629 

3253 
3774 
4110 
3576 
3531 

3443 
3184 
3570 
5140 
3386 

4008 
4867 
.5063 
4731 

4857 

3453 
3395 
3425 
3362 
3176 

3383 
4754 
4549 

3780 

3881 
3940 

3867 
139 

4103 
3802 
3687 

41^0 
4317 
4600 
.3461 
3939 

.3456 
33.33 
4213 
3905 
3926 

3796 
4093 
4626 
4669 
3975 

6125 

5484 
5360 

5,S0S 
5645 

3589 
41.36 
4080 
.3415 
3585 

3074 
6197 
6339 

3878 
4481 
4389 

4349 
137 

4256 
3665 
3267 

4079 
4715 
4617 
4715 
3640 

4135 
3542 
4061 
3934 
4199 

4708 
38.33 

3,S31 
3342 

5937 
6567 
5422 
6213 
5373 

3133 

3463 
3559 
3197 
3732 

4299 
4781 
4619 

4062 
4474 
4316 

4384 
143 

4181 
45S6 
3959 

4014 
43S5 
5765 
3905 
4661 

3795 
4439 
3634 
3928 
5047 

4038 
4176 
4376 
3714 
4440 

5766 
6567 
6798 
5054 
6033 

3343 
3526 
4180 
4036 
3450 

3699 
5940 
6330 

4359 
4458 
4505 

4407 
143 

4254 
4543 
3780 

4474 
3994 
4575 
4355 
4318 

4067 
4a36 
4166 
4119 
4294 

4338 
4331 
4205 
4333 
3863 

5033 
5796 
5837 
5.364 
5643 

5031 
3769 
■3913 
3780 
3729 

3931 
6014 

5483 

4240 
4425 
4598 

4421 

140 

1874 

4529  4646  41,SS 
3793  J  34:55  ,  .3310 

4433  4341  3769 
4479  1  4083  ,  3907 
4479  4189  3T17 
4326  4518  4026 
4644  3949  3876 

.3906  3,867  '  3073 
4733  4213  3.523 
4076  4184  3861 
3655  4074  3237 
4354  ,  3740  4073 

3803  i  3.508  3680 
4133  4073  1  3653 
3949  3.SO6  3690 
3900  3948  3945 
3281  3506  3631 

5112  5502  i  4399 
5635  1  4451  :  4536 
4867  4957  5233 
4654  4960  3880 
4561  ;  4.541  4890 

4955  !  5397  3133 
3440  3551  3908 
3754  3765  3130 
34S9  3.3.55  3918 

149 

1876 

124 

1876 

1877 

147 
131 

1878 

1879 

150 
143 

1880 

3443  ■IR.HR 

143 

1881 

3654 
4.335 
3471 
4609 
5090 

4789 
4600 
4551 
3938 
4191 

3740 
6393 
6510 
5413 
6030 

6399 
4559 
4097 
3840 
3933 

3793 
3874 
6410 

4135 
4536 
4834 

4498 
145 

4362 
3567 

4054 
4273 
4439 

4831 
4407 

:^iH8 

3.S08 
3661 

4385 
6410 
6628 
5553 
7090 

7297 
3840 
3668 
3946 
4547 

4036 
4839 
6073 

4070 
4365 
5143 

4636 
163 

134 

1883 

1883 

1884 

1885 

133 
137 
135 
141 

1886 

139 

1887 

139 

1888 

1889 

138 
143 

1890 

137 

1891 ... 

1893 

165 
191 

1893 

193 

1894 

176 

1895..... 

1896 

186 
165 

1897  

134 

1898 

139 

1899   

134 

1900 

1901 

3715  3397 
3363  3689 

3304 
3163 

123 
139 

1903 

5698  1  fiOQK 

4606 
4731 

3696 
3870 
3807 

3791 
133 

165 

1903 

5115 

4334 
4189 
4230 

4351 
143 

5007 

4131 
4079 
4361 

4154 
134 

180 

Average 

1873-1883 

1883-1893. . . . 
1893-1903 

1873-1903 

Average  ) 

daily  V . . 
1873-1903  \ 

139 
145 
151 

145 

Table  LXVIII  shows  the  total  monthly  and  average  daily  wind  movement 
for  31  years,  from  1873  to  1903;  also  the  average  daily  movement  for  the 
entire  30  years  ending  1902.  The  figures  are  based  on  the  continuous  record 
of  a  self-registering  Robinson  anemometer. 


The  average  daily  movement  for  an  entire  year  has  been  as  low  as 
124  miles,  as  in  1875,  and  as  high  as  150  miles,  as  in  1878,  confining 
our  choice  of  limiting  values  to  the  period  from  1873  to  1890,  during 
which  the  elevation  of  the  anemometer  remained  practically  unchanged. 


a  es 

C  O 


••iia 


«  cj  ■*  ce  c5     ■*  eo  S'l  >o  a) 


i-i!M>-i      i-i     i-ii5>-ii-ira  1-1  i-i 


b-oa 


■I^Jl^"    -l^^5"r®     5'o3=,5     »°5®5 
S5h?;0<     S5aK,-<Sn      >^5ai-;-<Z      CZ^hi-; 


■^-^"^  3  33       IB  O 


©  «  ©  ®  4) 


^OQ^WZ 


^r^^ 


•PA 


a^BQ 


■Jia 


•ISA 


3^H?Q 


••Jia 


•PA 


a^^g 


■Jia 


PA 


a^BQ 


•JICT 


•PA 


•[9A 


lejBQ 


©JBQ 


■JRI 


■I-J  V 


emi 


■•iia 


•i^A 


aiBQ 


•ISA 


MBg 

■Jta 


QVBg 


M®^ 


SOtClOO         3D3CXX>— i 


ra'^cc'*:^      C'iC''!CJT!ec     jjcccc*: 


•  —  CSIOtO         0iX^}»O3D 


h  »5  cc  »o  cc 


p;  ci  cc  -^  »o     c-j  t 


I-  M  05  Xi  m        t-  ffi  •*  rt  t- 


^3"*"^        *OOlOCC 


zzzzz 


ZZZZ        Zx.x>'>^ 


'^zz 


*®A?2'^T*'      J??!r 


■M  '-*<  tc  ;c  Xi 


iC  tC  rj  tC'f** 


-*  t--H.-^ 


808 


Ztc    ZZ 


iC  ip  — «  ^j*  :C' 

xzzzi^i 


rioi-io      —rtc;  —  — •      -^-t^x- 


z'^'^^z 


:;S;fa; 


z:z''z 


»>  SI  re  »  cc      M  ?5  ?J  « 


cc  X  » lO  ?i      ^  X  «  ?'t  X      X  ?i  as  o  -H      0^1  ~i"ds 
15  ?j  ?i «  ?}     ei  ■*  •>*  M  »      r!  rj  II  s!  ?i      is  cc  cc  01 


2-8S 


o»o-^rrx      cc-^ooii      rcio-H^^t— 
CT  ^       ?)  I?       01       ?1  re  ??      ^  -H  g; 


ZZZMZ 


x-*oiO'*     -^csaj-^o     -*ooo-* 
«T!re->»<?i     ?j-^-<T!?}     c^i^jrere'T! 


K>-i, 


:  o  o  :£■       ^H  C5  ^  ?-l 


2-iZ 


ot—  rerex     o»o-*gi-^ 


g»-H  —I 


?!  ?;?!?! —I 


C5  s  :r  -^  o 


re»ret 


■  X     c^re  03i^ 


Zx-yiZ- 


=^?^Z^Z 
Z>:     z 


H>>:-C>: 


:kZ; 


:o       Oi-:=- 


682 


?CSl^^^^g5       lO^-tXC'lO        iOOSlXl-        CIdOCSX        »-i?i»oxre        IMCC: 


C-?     re     iM  re  ?} 


^zzS= 


ZIS^M^; 


Z''2;^!2; 


''ZOJOQ 


occcooo 


CO-^Ot-        Xt--*»OX        ^CCiOC'tO 


O^lOO       COt— <o-^ 


ei»ii-i'?i^^     "^rt?!-*-^     »irere-*T!     raTs^isisi     siO'ire^t 


ots 


x-^reo5rt     pro-*i.-^ 


CC-hX 


gj  re     cc  o  re  =c  C3     t—  t^  i—  c^  »o     i—  t- 


-^  -^       T!  ci       ?j  M  -^  ^ 


Z^^xZ 


t»ZZ 


Zzaiz' 


oqZ'^'^ 


joreo     jjreoreri 

1!  Till  re  re     MI!  I!?!  re 


Cire-*      x;o:or!:c      lOtccaocs      x-*ocD 


J!  5^  re  T!  »!        -^ 


re  re  I!  rj  -H 


-*  C5  CD  r* .  c^ 


i-l        •?}  ?!  -H 


'  ?!  ?!        lO  X  ^  3i  -^ 


3i  -^        O  »1  35  i-( 

-^  re  g7  gl  ^ 


■x.  -r.  ■/. ''  "rfl 


ZZzxx 


:ZZ    ZZrZ       rr 


I!?!!!?!-^        51— IT!*??!        !■-!■- 


CJ  gl  »1  -H  .-I        ■-.  g!  X  CC  SI 

-^  ?>  re         gi     c^  ?! 


1-— 1-*0       CSIT!: 


I-  10  ^  X  := 


^H         ?!?!: 


-S«-^     g^^-^ 


Z     m'^Z 


s- 


"530003     o<-rerere     cjoosjo 
reciJire^     SI -H  ?!?>?!     -^  SI  SI  SI  re 


SI  re  SI  SI  re 


s5?;ss?    8-96 


■*i-05«re     o>03:iore 


551-1  OS -«<eo 


-re:c»o»o      osrecocsto      ■^'^xx 


ZZ^ZZ 

ioxio-*o 

S!S>SISIg 


Z  "^  VI  >:  Z 


Z     Z  X  Z  Z  Z     7:  x  z  z '' 


•z''z' 


'^ZZz 


z 


ZZZZZ 


SI  1  -  o  o  re 


X3J'*  —  Si     siore-^ 


•"^  '"*'  "*'      **  i^  *^  p^  53      ^^  C2  *^  ^  ^      "^      ^  "'^  !>■      ^      ^  ^ 

zzz    zzzzz    zs^zz"    Z^^Z"    '''^M'' 


z    zzz 


j'«?«-n«? 


MSI  SI  SI     SI  SI  sire 


I-  O  10  ■*  =        10  lO  1-  s» 

re  •OS!  SI  re      SI  SI  sj  re 


:,-63 


re  SJ  s>  »o  SI 


^reo-*x      oS"U5i-      —X  SI  35:c      x-^sj:cs 


;-*siiO 


■/.zz''z 


J  §  ?;  i.-     ?,  ?, 


ZZZZZ     Z     ZZZ     '^'^'^'f^'^     '^m'^'^ 


zz 


?.?iii2;-' 
zi^zz 


JosisiS?!     SSwS-^     SreSSeS     SSeSSil       t'68 


-  =>    s  5  y  ? 


rsii 


iO¥'-  X  : 


^i^^ 


T681 


81,81 


S68I 


8881 


poer 


8681 


6i8I 


9681 


— «      -^?» ^c^i      ^TO gi  e^t  0^  ?t  o^ _c^_»r^Mcc : Tool 

JSKJ:^^     ?=^W==^    ^S^5=^    ^^W^       ^  ^ 


bo 


O  C 

tn  " 

•"  3 

-o  o 

5  ^ 

•r;  w 

^  M 

"S  CI  o 

W)  '"  'H 

T-l  Ci 

+J  <"   bp 

10  <B   '3 

t-  t-  ,3 

00  03  !2 

^  w   S 

a  Qj 

CM  O       O 

O     t- 

'^  «  .2 


(i5    aJ 


,d   3 

0)     O 


•g   2   -o 

QJ  fl  fS 
*-i    eS    0) 

>»  t>>  a 
111 
^  a.9 


^a 


»J3 


5  a  ^ 

5  0)  a 

03  ai-i 

*J 

►-1     ^  "O 

X  ^  iJ 

1-5    fe  3 

■— I     a)  ra 

o  rt 


358 


THE    CLIilATE    OF    BALTIMORE 


Maximum  Wind  Velocities. 
In  the  preceding  paragraphs  the  total  wind  movement  over  the  station 
for  an  entire  month,  and  the  average  hourly  and  average  daily  move- 
ment were  alone  considered.  In  Table  LXXII  a  record  will  be  found 
of  the  highest  velocity  of  the  wind  attained  in  any  5-minnte  period 
during  each  month  and  year  from  1875  to  1903,  together  with  the 
accompanying  direction  of  the  wind  and  the  date  of  occurrence.  In 
determining  the  maximum  wind  velocity  for  the  day,  the  sheet  contain- 


60 

■ 

- 

- 

40 

- 

- 

- 

- 

- 

- 

- 

- 

- 

- 

- 

- 

20 

0 

Fig.  72. — The  Frequency  of  Storm  Winds. 

The  diagram  shows  the  variations   in   the  annual  frequency  of  winds  exceeding  25 
miles  per  hour. 


ino^  the  continuous  record  of  the  anemometer  is  examined  and  the  five- 
minute  interval  selected  during  which  the  velocity  is  greatest.  The 
number  of  miles  or  fractions  of  a  mile  registered  during  this  5-minute 
period  is  then  multiplied  by  12  in  order  to  obtain  the  rate  of  movement 
per  hour,  or  what  is  usually  termed  the  hourly  velocity  of  the  wind.  All 
of  the  daily  records  since  1875  have  been  carefully  examined  and  the 
highest  velocity  recorded  during  each  month  selected  and  entered  in 
Table  LXXII,  at  the  same  time  noting  the  date  of  occurrence  and  the 
direction  of  the  Avind  during  the  selected  5-minute  period.  An  examina- 
tion of  the  table  shows  that  high  winds  are  not  confined  to  any  particular 
season  of  the  year,  but  have  occurred  in  all  months.     The  high  winds  of 


MARYLAXD    WEATHER    SERVICE  259 

the  winter  months  occur  in  connection  with  the  well-defined  cyclonic 
disturbances,  while  the  high  velocities  of  the  summer  months  accom- 
pany the  thunderstorms,  or  the  tornado,  in  the  rare  instances  of  its 
occurrence  in  this  vicinity.  The  annual  variations  of  the  maximum 
velocity  are  shown  in  Fig.  T2. 

The  highest  velocity  of  the  wind  recorded  at  the  Baltimore  Station 
of  the  U.  S.  Weather  Bureau  since  18T5  occurred  during  the  storm  of 
July  20,  1902,  when  the  wind  blew  at  the  rate  of  TO  miles  per  hour  for 
five  minutes.  Further  particulars  of  this  storm,  which  was  one  of  the 
most  destructive  ever  visiting  this  vicinity,  will  be  found  in  a  later 
section  of  this  report.  Selecting  the  highest  recorded  velocities,  in  miles 
per  hour,  for  each  month  of  the  year,  we  have  the  following  comparative 
figures : 

HIGHEST  MONTHLY  VELOCITIES. 


> 

6 

o 

« 

Z. 

Q 

48 

5-t 

S 

E 

1891 

1898 

28 

4 

28 

30 

Highest  vel....  48         45  50     ,     fiO  4.S         i-2  70  45  38         45  48  ,     54  70 

Direction W      NW       S  NW  |    W       SW  W  SW  NW     SW  S         E  W 

Year 1894  1893  1896  1879  1893  i  1893  1902  '  1888  I  1892  1878  \   1891  |  1898  1902 

Day .30    19  19    3  23  I  27  20  8  ■  26  | 

Av.vel.of  ma.\.-  29  ,  30  30  i  30  26  1  26  28  24  24    27  28    30  28 


The  average  of  all  maximum  velocities  during  the  28  years  from  1875 
to  1902,  as  shown  in  the  last  line  of  the  above  table,  indicates  a  remark- 
ably uniform  value  for  this  factor,  throughout  the  year.  The  highest 
monthly  average  velocity  (30)  differs  from  the  lowest  (24)  by  only 
6  miles.  The  lowest  velocities  occur  in  August  and  September,  and  the 
highest  in  February,  March,  April  and  December.  The  fact  that  the 
September  records  show  the  lowest  average  velocities  for  storm  winds 
is  significant  in  view  of  the  popular  association  of  the  so-called  "  Equi- 
noctial "  storms  with  this  month. 

As  already  stated,  wind  velocities  are  generally  expressed  in  terms  of 
the  rate  per  hour  based  upon  the  actual  velocity  during  a  five-minute 
period.  By  basing  the  rate  per  hour  upon  the  duration  of  the  mile 
made  in  the  shortest  time,  we  obtain  what  is  officially  designated  as  the 
extreme  velocity.  By  this  method  we  are  more  liable  to  obtain  the 
18 


260 


THE    CLIMATE    OF   BALTIMORE 


velocity  in  brief  gusts  of  wind,  velocities  which  are  lost  when  the  hourly 
rate  is  based  upon  the  movement  during  a  period  of  five  minutes.  As 
much  of  the  destruction  due  to  high  winds  is  wrought  during  these  brief 
gusts,  or  squalls,  the  extreme  velocity  is  a  factor  of  great  importance.  It 
is,  in  nearly  all  cases,  higher  than  the  maximum;  it  cannot  be  lower. 
There  is  no  fixed  relation  between  the  two  velocities ;  it  may  be  of  inter- 
est, however,  to  show  to  what  extent  they  have  differed  from  one  another. 
Basing  our  inquiry  upon  the  official  record  of  the  monthly  maximum  and 
extreme  velocities  during  the  period  from  1888  to  1903,  we  have  the 
following  comparative  figures: 

RELATION   BETWEEN   MAXIMUM   AND   EXTREME   VELOCITIES. 
(In  miles  per  hour.) 


Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Deo. 

Maximum 

48 
13 

45 
55 

50 
60 

42 
50 

4.3 

48 

42 

50 

70 
75 

45 
52 

38 
50 

12 

43 
50 

8 

48 
60 

12 

54 

60 

10 

10 

8 

5 

8 

5 

7 

6 

This  relationship  may  be  expressed  b}'  another  method.  In  place  of 
selecting  the  highest  maximum  and  highest  extreme  velocities  for  each 
month,  we  may  examine  all  cases  of  high  winds  occurring  in  a  stated 
time  and  note  the  difference  between  the  maximum  and  extreme  veloci- 
ties. This  has  been  done  for  a  period  of  three  years  with  the  following 
result : 

DIFFERENCES  BETWEEN  MAXIMUM  AND  EXTREME  VELOCITIES. 
(In  miles  per  hour.) 


Jan.    Feb.  Mar.  Apr.   May  .lune  July  Aug.  Sept.  Oct.    Nov.  Dec.  Year 


Average  diff 4.5 


Greatest  " 
Least  " 
No.  of  cases. 


4.5 

10 

1 

29 


4.3 


4.3      3.4 

5 

3 

11 


3.6 
12 
0 
14 


7.8  3.9 

17    I  10 

I 

1  1 

15  7 


4  8 

16 

1 

13 


3.6 

10 

1 

17 


5.0 


4.2 


4.5 
17 
0 
195 


The  highest  wind  velocities  generally  occur  in  connection  with  a 
northwest  wind  in  all  months  of  the  year.  These  winds  usually  accom- 
pany a  rising  barometer  and  occur  a  short  time  after  the  shift  in  tlie 


MARYLAND    AVEATHER    SERVICE 


261 


wind  which  follows  the  turn  in  the  barometer.  While  northwest  is  the 
usual  direction  of  the  storm  wind,  all  directions  of  the  compass  are  rep- 
resented. In  Table  LXXII  there  are  348  records  of  high  winds  covering 
a  period  of  29  years;  placing  these  in  the  order  of  frequency  of  the 
directions  from  which  they  came,  we  have  the  following  relative  positions 
for  the  entire  year : 

RELATIVE  FREQUENCY  OF  HIGH  WINDS. 


Direction  of  wind   

NW 

W 

SW 

NE 

N 

SE 

E 

S 

Percentage  of  frequency  

41 

20 

12 

8 

1 

4 

4 

4 

The  same  order  of  frequency  obtains  practically  in  all  months  of  the 
year.  In  nearly  three-fourths  of  all  instances  of  storm  winds,  the  direc- 
tion is  from  some  point  between  southwest  and  northwest.  In  only  12 
per  cent  of  instances  is  the  direction  from  some  point  between  east  and 
south.  High  winds  from  the  north  or  from  the  east  are  of  compara- 
tive] v  rare  occurrence  at  Baltimore. 


Frequency  and  Duration  of  Stated  Wind  Velocities. 

The  hourly  wind  velocities  during  a  period  of  five  years  (namely, 
from  1893-96  and  1903)  were  tabulated  into  groups  in  order  to  deter- 
mine the  relative  frequency  of  stated  velocities.  The  result  is  shown  in 
the  following  table,  in  which  the  frequencies  are  expressed  in  terms  of 
percentages  of  the  total  number  of  hours  in  each  month  : 
frequency  of  stated  wind  velocities. 

(In  percentage  of  possible  number  of  hours  per  month.) 


Miles  per  hour.. 

Jan. 

Feb. 

Mar.  Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

41.8 

Nov. 

Dec. 

An'l 

0  5 

37.6     31.2 

33.7     28.8 

35.3 

41.5 

44.3 

49.0 

45.3 

39.3 

40.7 

39.1 

ti-10 

35.7     .35.2 

3(i.4     40.7 

4.3.0 

44.4     43.1     39.7     38.5 

.37.6 

35.6 

.35.3 

38.8 

11-15 

15.8     15.7 

16.2      19.9 

15.6 

12.2     10.8       9.1      13.2 

13.8 

16.1 

15.7 

14.5 

16-20 

6.4  ,    9.9 

8.3  .     8.2 

4.9 

1.8 

1.3      1.3      2.5 

4.6 

6.5 

6.3 

5.2 

21-25 

2.8  1    4.9 

3.5  1    1.9 

O.T 

0.1 

0.1      0.3      0.4 

1.7 

2  2 

1.5 

1.6 

26-30 

1.2       2.1 

1.4 

0.5 

0.4 

0.1 

0.2 

0.2 

0.3 

0.2 

0.4 

0.6 

.31-40 

0.3  :     0.9 

0.3 

0.06 

0.1 

0.06 

0.1 

0.2 

0.1« 

0.1 

41-50 

0.03'  .... 

0.03 

0.0 

262 


THE    CLIMATE    OF    BALTIMORE 


Winds  of  10  miles  per  hour  and  under  prevail  during  about  78  per 
cent  of  the  total  number  of  hours  of  the  year;  winds  of  11  miles  to  20 
miles  during  less  than  20  per  cent.  Hence  the  total  duration  of  veloci- 
ties exceeding  20  miles  per  hour  is  only  about  2.3  per  cent  of  the  entire 
year,  or  about  eight  and  a  third  days.  Storm  winds,  or  winds  exceeding 
25  miles  per  hour,  prevail  during  about  62  hours  in  an  average  year. 


Average  Duration  of  Storm  Winds. 

It  will  be  seen  from  the  statements  in  the  preceding  paragraph  that 
winds  having  a  velocity  exceeding  25  miles  per  hour  are  of  brief  duration. 
The  duration  decreases  rapidly  with  increase  in  wind  velocity.  The  rate 
of  decrease  may  be  readily  judged  from  the  figures  in  the  above  table 
representing  the  annual  relative  frequency  of  stated  velocities.  Basing 
our  calculations  upon  the  same  period  of  five  years  employed  in  deter- 
mining the  relative  frequency  of  stated  velocities,  v^e  obtain  some  inter- 
esting figures  defining  the  average  duration  of  storm  winds. 


AVERAGE 

DURATION  OF 

STORM  WINDS. 

(In  hours  and  minutes.) 

1-5 

i 

03 

b 
< 

>> 

03 

0) 

c 

3 

1-5 

>> 

be 
< 

P. 

o 
O 

> 
o 
I? 

6 

OS 

>> 

Total  annual 

duration . . . 

23.36 

43.35 

28.25 

12.25 

5.35 

0.26 

0.60 

4.10 

3.40 

11.10 

9.50 

13.06 

1.56.40 

Averag-e 

frequency.. 

6.2 

9.2 

7.6 

5.0 

4.4 

2.0 

2.6 

1.4 

2.6 

4.2 

4.8 

4.4 

54.4 

Duration  per 

storm 

3.50 

4.40 

3.40 

2.30 

1.20 

0.12 

0.20 

3.00 

1.25 

2.40 

2.00 

3.00 

2.50 

Greatest 

duration  — 

19.00 

46.00 

23.00 

16.00     7.00 

.36 

.30 

18.30 

7.00 

9.10 

6.00 

19.00 

46.00 

In  the  winter  and  early  spring  months  a  storm  wind  usually  continues 
from  three  to  four  hours.  The  duration  rapidly  diminishes  on  the 
approach  of  summer,  reaching  a  minimum  in  June  and  July,  when  the 
average  duration  is  only  a  few  minutes.  The  high  winds  of  summer 
usually  occur  in  connection  with  thunder  squalls  of  brief  duration, 
while  those  of  winter,  spring  and  fall  accompany  the  passage  of  well- 
defined  cyclonic  storms.  The  comparatively  long  duration  of  August 
storm  winds  in  the  above  table  is  due  entirely  to  the  severe  gulf  storm  of 
August  28-29,  1893,  during  which  the  wind  blew  a  gale  for  many  hours. 


MARYLAND    WEATHER   SERVICE  263 

a  duration  which  would  be  considered  long  even  for  a  winter  gale  of  the 
severest  type.  Xeglecting  this  storm,  the  August  average  duration  for 
the  remaining  four  years  is  about  35  minutes. 

During  the  passage  of  the  Gulf  storm  of  February  7-10,  1895,  the 
wind  blew  at  Baltimore  with  a  velocity  exceeding  25  miles  per  hour  for 
about  46  consecutive  hours.  It  then  fell  below  the  storm  velocity  for 
about  12  hours  and  again  went  above  25  miles  per  hour  for  another 
period  of  12  hours.  It  is  one  of  the  longest  storm  periods  on  record  at 
Baltimore.  The  storm  originated  in  the  Gulf  of  Mexico  on  the  6th, 
and  moved  rapidly  eastward  and  northward  along  the  Atlantic  coast 
from  Florida  to  the  Gulf  of  St.  Lawrence,  the  center  passing  just  east- 
ward of  Baltimore  on  the  8th,  with  a  maximum  velocity  of  42  miles  per 
hour  from  the  west.  The  barometric  gradient  between  the  center  of  the 
storm  and  the  center  of  the  area  of  high  pressure  to  the  west  and  north- 
west was  very  great  throughout  its  course,  amounting  at  one  time  to 
about  two  and  a  half  inches.  The  extreme  velocity  of  50  miles  per 
hour  was  reached  on  the  8th  at  about  noon. 

As  the  summer  high  winds  occur  mostly  in  connection  with  thunder- 
storms, their  time  of  greatest  frequency,  and  hence  greatest  probability, 
is  from  3  p.  m.  to  4  p.  m.  The  winter,  spring  and  fall  storm  winds, ' 
accompanying  cyclonic  disturbances  which  occur  at  any  hour  of  the  day, 
also  have  a  well-marked  tendency  to  fall  within  the  early  afternoon 
hours.  This  may  easily  be  explained  by  supposing  that  the  cyclonic 
winds  are  augmented  at  these  hours  to  a  maximum  extent  by  the  diurnal 
wind  movement. 

Gales. 

A  gale  is  technically  defined  by  a  wind  velocity  of  40  miles,  or  over, 
per  hour.  Such  winds  have  been  recorded  on  42  occasions  at  the  Balti- 
more office  of  the  U.  S.  Weather  Bureau  since  1873.  They  have  been  of 
comparatively  great  frequency  in  some  years,  notably  in  1893,  which  is 
credited  with  nine;  there  were  seven  in  1903.  In  half  the  years  since 
1873,  none  were  recorded.  The  highest  velocity  registered  in  the  years 
from  1880  to  1887  was  39  miles.     As  is  the  case  with  most  high  winds, 


364 


THE    CLIMATE    OF    BALTIMORE 


TABLE  LXXIII.— SUMMARY  OF  WIND  VELOCITIES. 
(1873-1902.) 


January . . 
February. 

March 

April  

May 

June 


July  

Aug'ust  — 
September. 

October 

November. 
December  . 


Year 6.1      8.0 


Means. 


6.0 
6.7 
7.3 
6.9 

6.3 

6.9 

5.6 

5.1 
5.4 
5.7 
6.0 
5.9 


10.8 
10.8 
9.3 

8.9 

7.8 

7.4 

7.0 
7.1 
8.2 
9.1 
8.1 


1893 


1891 


1891 
1893 
1895 


3.7 
3.9 
5.3 
5.0 

4.9 

4.5 

4.5 

3.9 
4.4 
4.1 
4.4 
4.0 


1877 
1877 
1875 
1900 


1901 

1899 

1897 
1900 
1901 
1896 
1875 


5.1     1900      28 


Maxima. 


c3  sj 


1879 
1893 
1893 

1903 

1888 
1893 
1878 
1891 


70   1903   14 


1881 
1883 
1901 
1885 
1891 
1900 

1881 

1884 
1886 

1890 
1900 

1883 

1881 

1883 

1898 
1880 


Storm  winds.* 


as 

2-5 

sa 

4.4 

10 

5.5 

12 

6.7 

13 

5.3 

13 

3.9 

9 

3.3 

5 

1.8 

4 

1.2 

4 

1.4 

5 

3.0 

7 

3.7 

9 

3.9 

8 

42.0 

70 

1878 

1895 

1881 

1880 

(1878 
'11893 
11877 
11879 
11878 
-^1896 
(1901 

1887 
(1889 
11896 

1894 

1886 
j  1885 
11887 


*  Winds  exceeding  25  miles  per  hour. 


gales  blow  mostly  from  the  northwest  or  west.  Of  the  42  instances 
referred  to  above,  the  relative  frequency  of  the  points  of  the  compass 
from  which  they  blew  is  as  follows : 


DIRECTION   OP   THE   WIND   IN   GALES. 


NW 

W 

S 

SW 

SE 

NE 

N 

E 

Total 

Number  of  gales. . . 

15 

13 

4 

3 

2 

3 

3 

1 

42 

The  distribution  of  gales  by  months  shows  that  they  have  been  most 
frequent  in  February.  The  month  of  "  equinoctial  storms  "  is  the  only 
month  without  a  gale  to  its  credit  in  30  years. 


FREQUENCY  OF  GALES  IN  30  YEARS. 

Jan. 

Feb. 

Mar. 
3 

Apr. ;  May  June  July 

Aug. 

Sept. 

Oct.  j  Nov.  Dec. 

Year 

2 

10 

2 

2 

3 

4 

2 

0 

3          5 

6 

43 

MAKYLAN'D    WEATHER    SERVICE 


265 


Prevailing  Hourly  Wind  Directions. 

We  have  seen  in  preceding  paragraphs  that  there  is  a  well-defined 
diurnal  fluctuation  in  the  velocity  of  the  wind.  Without  a  close  obser- 
vation of  diurnal  changes  of  direction  in  the  locality  of  Baltimore,  a  well- 
marked  periodicity  would  scarcely  be  suspected.  Such  is  the  fact,  how- 
ever, as  demonstrated  by  a  reduction  of  the  hourly  observations  for  a  period 
of  ten  years.  The  results  are  shown  statistically  in  Table  LXXIY  and 
graphically  in  Plate  XI,  Fig.  73.     A  well-defined  diurnal  period  was 

TABLE  LXXIV.-PREVAILING  HOURLY  WIND  DIRECTION. 


Hours 

a 

S3 

1-5 

i 

i 

a. 

>> 

03 

c 

>. 

sib 

3 

*> 

u 

o 

1 

pc 

^ 

< 

s 

►-5 

i-s 

< 

CQ 

O 

(z; 

P 

"^ 

lA.M 

NVT 

w 

NW 

W 

w 

SW 

sw 

sw 

sw 

N 

w 

W 

W 

o 

sw 

w 

NW 

w 

w 

SW 

sw 

sw 

NW 

N 

w 

w 

SW 

3 

sw 

w 

NAV 

NW 

SW 

sw 

sw 

N 

NW 

NW 

w 

NW 

NW 

4 

w 

NW 

W 

NW 

NW 

sw 

sw 

NW" 

N 

NAV 

NW 

w 

NAV 

5 

w 

W 

W 

W 

NAV 

sw 

sw 

NW 

N 

N 

NW 

w 

AV 

6 

w 

W 

W 

W 

N 

sw 

sw 

NW 

N 

NW 

N 

W 

W 

NW 

NW 

W 

W 

NE 

sw 

sw 

N 

N 

N 

NW 

AV 

NW 

8 

W 

W 

W 

E 

N 

sw 

sw 

N 

N 

JN 

W 

W 

AV 

9 

W 
W 

w 

SAV 
NAV 

N 

N 

SW 

SW 

sw 

N 

sw 
sw 

SW 

SW 

NE 
E 

NW 
E 

N 
N 

W 

SAV 

SW 

10 

SW 

11 

w 

w 

E 

SE 

SR 

w 

sw 

N 

N 

E 

SW 

SW 

SW 

w 
w 

w 
w 

SE 
SR 

SE 
SE 

SE 
SE 

SE 
SE 

sw 

SE 

SE 

SE 

E 

SE 

E 

SE 

w 
sw 

NAV 
NAV 

SE 

1  P.M 

SE 

2 

w 

NW 

SE 

SE 

SR 

SE 

sw 

SE 

SR 

SE 

w 

A\" 

SE 

3     ... 

w 
w 

W 
NW 

SE 
SE 

SE 
SK 

SE 
SR 

SE 
SR 

SE 
SR 

SE 
SE 

SE 
SR 

SE 
SE 

sw 

SE 

SW 
AV 

SE 

4 

SE 

6 

w 

W 

SE 

SR 

SE 

SE 

SE 

SE 

SR 

SR 

SE 

AV 

SE 

fi 

w 

W 

SE 

SE 

SE 

SE 

SW 

SE 

SR 

SK 

SR 

AV 

SE 

"r 

NW 
W 

W 
W 

E 
W 

SE 
SE 

SE 
SR 

SE 
SE 

SW 

sw 

SE 
SR 

SE 
SE 

SE 
SE 

SE 

w 

NAV 
AV 

SE 

S 

SE 

9  

w 

NW 

NW 

SE 

SR 

S 

sw 

SW 

SW 

E 

w 

NW 

SW 

10 

\w 

W 

NW 

SE 

SR 

SW 

sw 

SW 

SW 

K 

w 

AV 

sw 

11 

NW 

W 

NW 

SR 

SR 

sw 

sw 

SW 

SW 

NW 

w 

NW 

sw 

Midnig-ht 

SW 

W 

NW 

W 

SW 

sw 

sw 

SW 

SW 

N 

N 

W 

sw 

Prevail,  direction. . 

W 

W 

NW 

SE 

SE 

sw 

sw 

SE 

SE 

SE 

w 

W 

SE 

Table  LXXIV,  showing  the  prevailing  direction  of  the  wind  at  the  hours 
stated  in  the  first  column,  is  based  upon  a  record  of  ten  years,  extending 
from  1893  to  1902. 


not  at  first  expected  to  be  revealed  by  the  average  hourly  values  which 
included  all  the  observations  of  the  year,  or  e\'en  all  of  any  particular 
month.  Hence  the  first  attempt  to  detect  a  periodic  movement  was 
made  by  selecting  days  in  the  months  of  January,  April,  July  and  Octo- 
ber, during  which  the  skies  were  prevailingly  clear,  and  the  wind  move- 
ment was  light.     This  was  done  with  a  view  to  eliminating  the  influence 


266  THE    CLIMATE    OF    BALTIMORE 

of  neighboring  .cyclonic  disturbances.  The  result  of  such  classification 
is  shown  in  Fig,  74  for  January,  the  diagram  being  based  on  ten  selected 
days  during  which  the  sunshine  exceeded  90  per  cent  of  the  possible 
amount,  and  the  wind  movement  was  less  than  100  miles.  The  wind 
direction  observations  were  classified  into  morning  and  afternoon  winds, 
the  former  class  including  the  hours  from  midnight  to  noon,  the  latter 
from  noon  to  midnight.  A  prevailing  westerly  wind  during  the  morn- 
ing hours  and  an  easterly  wind  during  the  afternoon  hours  was  so 
clearly  revealed  in  all  months  in  these  diagrams  that  the  hourly  obser- 
vations for  each  hour  and  for  the  entire  period  of  ten  years  were  tabu- 
lated and  charted,  with  the  result  shown  in  Table  LXXIV  and  Fig.  73. 
These  tables  and  diagrams  reveal  some  interesting  and  probably  unsus- 
pected facts  concerning  the  daily  fluctuations  in  the  wind  direction  at 
Baltimore.  A  well-defined  diurnal  periodicity  appears  in  all  seasons 
of  the  year  when  the  local  conditions  are  not  influenced  by  the  presence  of 
cyclonic  disturbances.  This  is  quite  as  well  marked  on  cold  winter  days  as 
in  the  summer  time.  Even  by  employing  all  observations,  the  average 
of  all  conditions  of  the  weather,  this  periodic  movement  is  conspicuous 
excepting  in  the  winter  months  of  December,  January  and  February, 
when  the  cyclonic  winds  almost  completely  mask  the  periodic  movement. 
An  examination  of  Fig.  73  shows  a  prevailing  wind  from  some  quarter 
between  northwest  and  southwest  at  all  hours  between  midnight  and 
11  a.  m.,  with  a  very  few  exceptions  when  they  are  from  the  north.  This 
is  true  for  all  months  of  the  year.  In  January,  February  and  December 
these  westerly  winds  continue  throughout  the  day.  In  all  other  months 
there  is  an  abrupt  change  in  the  direction  to  the  southeast  about  noon; 
a  little  earlier  in  March,  April  and  October  and  a  little  later  in  July 
and  November.  The  southeast  wind  then  continues  without  interruption 
to  an  early  evening  or  a  night  hour,  when  the  direction  returns  quite 
as  abruptly  to  the  southwest  or  west.  The  hour  of  return  to  the  morn- 
ing direction  varies  more  than  the  change  from  the  morning  to  the 
afternoon  direction.  The  southeast  returns  to  southwest  in  July  as 
early  as  6  p.  m. ;  in  April  and  May  as  late  as  11  p.  m.  The  southeast  or 
afternoon  direction  is  maintained,  accordingly,  for  a  minimum  period 


MARYLAND  WEATHER   SERVICE. 


VOLUME  2,   PLATE  XI. 


o       z       o 


y- 


"^  1  "l  >l  <  1  >|r 


vl^^^-Sr^^^^r^ 


HH^ 


^'-n^'-^- 


^^^^r 


v?^ 


-?^^^^^^ 


i^ 


)(}()( k  k  k  A  A  \  >{ 


^r 


\  <  1- 


-4-^^- 

^s^ 


v^^ 


-^^ 


%  Y  ""'^  ""^ 


V*^ 


v'^ 


V'^ 


^^ 


ri^-A- 


^^ 


v*-v^ 


^^ 


V- 
V- 


s      r? 


.2      M 

a 
>5  ■- 


>»    .a  bij 

S         a;    ■— 


2   -^ 


71       OJ 


2    3 


MARYLAND    WEATHER    SERVICE 


267 


of  5  hours,  as  in  July,  to  a  maximum  of  13  hours,  as  in  April,  May  and 
October.  For  the  year  as  a  whole,  the  southwest  wind  changes  to  a 
southeast  at  noon,  maintains  this  direction  until  8  p.  m.,  and  then 
returns  to  the  southwest.  The  southwest  becomes  a  west  or  northwest 
wind  from  1  a.  m.  to  8  a.  m.,  and  then  southwest  again  from  9  a.  m.  to 
11  a.  m.  These  hourly  changes  are  surprisingly  uniform  throughout 
the  year  when  prevailing  directions  for  a  long  period  are  considered,  or 
on  quiet  days,  for  short  periods  of  only  a  few  days. 


Fig.  74. — Prevailing  Morning  and  Afternoon  Wind  Directions  in  January. 

The  heavy  black  lines  Indicate  the  prevailing:  winds  during  the  hours  from  midnight 
to  noon  ;  the  light  lines  show  the  prevailing  winds  during  the  hours  from  noon  to  mid- 
night on  selected  days  in  January  with  a  light  wind  and  bright  sunshine. 

In  the  figure  based  on  the  rougher  grouping  into  morning  and  after- 
noon directions,  the  percentage  of  frequency  of  occurrence  of  the  wind 
from  each  quarter  is  also  shown.  Fig.  74  indicates  that  even  in  mid- 
winter, represented  by  the  month  of  January,  the  morning  winds  are 
distinctly  west  of  the  north  and  soutli  line,  and  the  afternoon  winds 
mostly  to  the  east.  In  Fig.  73,  which  is  based  on  all  observations  during 
a  period  of  ten  years,  the  winds  are  westerly  in  January,  February  and 
December,  both  morning  and  afternoon,  as  stated  above.  A  feature 
especially  worthy  of  note  is  the  abrupt  change  from  southwest  to  south- 


368  THE    CLIMATE    OF   BALTIMORE 

east  about  midday.  The  change  from  northwest  or  west  to  southeast, 
and  in  the  reverse  order,  is  made  without  lingering  in  the  south.  A 
prevailing  south  wind  is  not  revealed  in  the  diagrams  or  table  for  even 
an  hour  in  any  month  of  the  year. 

It  is  difficult  to  assign  a  satisfactory  cause  for  this  daily  periodic 
movement  in  the  vicinity  of  Baltimore.  The  first  explanation  which  is 
suggested  is  that  it  is  a  land  and  sea  breeze  effect.  The  station  is,  how- 
ever, too  far  removed  from  a  body  of  water  sufficiently  large  to  produce 
the  effect,  even  at  the  season  of  the  year  when  contrasts  in  temperature 
between  land  and  water  are  strongest.  The  harbor  presents  a  compara- 
tively small  water  area  in  Patapsco  Eiver,  which  is  in  turn  twelve  to 
fifteen  miles  from  Chesapeake  Bay,  while  the  station  is  fully  a  mile 
distant  from  the  harbor.  These  facts  of  local  conditions  make  it  ex- 
tremely improbable  that  the  winds  are  the  effect  of  an  interchange  of 
air  between  land  and  water  areas.  The  suggestion  arises  whether  the 
fluctuations  are  an  integral  part  of  the  diurnal  cyclone  described  in  the 
preceding  section  on  pressure  changes.  To  demonstrate  this  would 
require  a  similar  discussion  of  the  hourly  changes  in  direction  at  many 
widely  scattered  stations,  especially  at  points  somewhat  nearer  the  path 
of  the  center  of  the  diurnal  cyclone,  and  on  both  sides  of  the  equator. 

Prevailing  Monthly  and  Annual  Directions. 
In  view  of  what  has  been  presented  in  the  foregoing  paragraphs  con- 
cerning the  hourly  changes  in  the  direction  of  the  wind,  it  becomes 
obvious  that  the  choice  of  hours  of  observation  is  an  important  matter 
in  determining  the  prevailing  monthly  and  annual  direction  of  the  wind 
at  any  given  locality.  Most  systems  of  observations,  before  the  days  of 
continuously  recording  instruments,  provided  for  three  eye  observa- 
tions :  one  about  7  in  the  morning,  another  about  3  in  the  afternoon, 
and  the  third  at  about  9  in  the  evening.  This  combination  yields  a  very 
fair  value  for  the  average  direction  for  the  day.  The  prevailing  direc- 
tions based  on  two  daily  observations  from  1893  to  1903  are  placed 
alongside  of  the  prevailing  directions  computed  from  three  daily  obser- 
vations and  from  hourly  observations  covering  the  same  period.     The 


Fig.  75. — Relative  Frequency  of  Prevailing  "Wind  Directions. 

The  diagram  shows  the  relative  froquency  of  the  prevailing  directions  of  the  wind  in 
the  months  of  January,  April,  July  and  October,  and  in  the  year.  For  example,  for  the 
month  of  July,  the  prevailinK  directions  during  a  period  of  ten  years  were  confined  to 
southwest  and  southeast  winds  ;  in  January,  the  prevailing  winds  were  always  westerly 
during  the  same  period,  etc. 


270 


THE    CLIMATE    OF   BALTIMORE 


WARM    Year. 

NORMAL     YEAR. 

Colo     Yt'- 

1893-4 

DEC              JAN              FEB 

1903-4 

\ 
\ 
\ 

1903 

\ 
\ 

\ 

1893 

\ 

1900 

JULV       y'V     AUG. 

JUNE  y^ 

1903 

1900 

Kr., 

\_ 

1890 

D 

N    ,        D 

1893 

J 

Fig.  76. — Prevailing  Monthly  Directions  of  the  Wind  in  Warm,  in  Normal 
and  in  Cold  Seasons  and  Years. 


differences  are  marked  only  in  August,  September  and   October.     By 
the  system  of  two  daily  eye  observations  we  obtain  a  prevailing  north 


MARYLAND    WEATHER    SERVICE 


271 


wind  in  September  and  northwest  in  Octol^er,  whereas  the  hourly  obser- 
vations show  a  prevailing  direction  from  the  southeast  during  both 
months.  The  resultant  prevailing  directions  based  on  three  daily 
observations  agree  somewhat  more  closely  with  those  derived  from  hourly 
observations,  the  chief  divergence  occurring  in  August  and  October. 
The  annual  path  pursued  is  best  represented  by  the  diagram  in  Fig.  76, 
which  is  based  on  24  hourly  observations. 

The  prevailing  monthly  directions  derived  from  the  three  series  of 
observations  are  as  follows: 

PREVAILING  DIRECTIONS. 


Jan. 

Feb. 

Mar. 

Apr 

May 

June 
SW 

sw 

SW 

July 

SW 
SW 
SW 

Aug 

Sept. 

Oct. 

Nov. 

Dec. 

Year 

7  a.      3 }). 

8  a.     8  p. . 

3P 

W 
W 
W 

W 
W 
W 

W 
NW 

SB 
SB 

SB 
SB 
SB 

SB 
SW 
SW 

SB 

N 
SB 

N 

NW 
SB 

W 

NW 
W 

W 

NW 
W 

W 

NW 

Hourly  ob 

ser%'t'ns 

NW 

SB 

W.SE 

There  is  a  fair  degree  of  uniformity  from  3'ear  to  year  in  the  prevail- 
ing directions  of  the  wind  for  the  same  months.  The  extent  of  the 
departure  from  the  average  direction  is  indicated  in  Fig.  76,  in  which 
the  prevailing  directions  are  shown  for  seasons  and  a  year  with  a  normal 
temperature,  a  well-marked  temperature  below  the  normal  and  for  those 
well  above  the  normal  in  temperature.  In  each  case  these  seasons  and 
years  have  been  selected  from  the  period  from  1893  to  1904,  and  hence 
the  prevailing  directions  are  based  on  hourly  observations.  An  inspec- 
tion of  the  figure  will  show  that  in  nearly  all  cases  there  is  an  unusual 
percentage  of  northwest  winds  in  the  cold  seasons,  and  a  predominance 
of  southerly  winds   (southwest  to  southeast)   in  the  warmer  seasons. 

This  is  in  harmony  with  the  results  obtained  by  determining  the 
average  temperature  of  winds  from  each  quarter.  Selection  was  made 
of  a  number  of  days  in  each  of  the  months  of  January,  April,  July  and 
October,  during  which  the  wind  blew  from  the  same  quarter  all  or  most 
of  the  day.  This  was  repeated  for  each  of  the  eight  points  of  the  com- 
pass. The  average  temperature  of  these  days  was  when  computed  from 
the  hourly  observations.  It  was  not  always  possible  to  find  days  during 
which  the  wind  blew  from  the  same  quarter  more  than  half  the  day;   in 


272 


THE    CLIMATE   OF   BALTIMORE 


Buch  cases  it  was  necessary  to  admit  days  with  a  direction  45°  on  either 
side  of  the  desired  point  of  the  compass. 

In  the  winter  months,  the  southeast  winds  are  the  warmest;    in  the 


TABLE  LXXV.- 

PREVAILING  MONTHLY  AND  ANNUAL  DIRECTION  OF  WIND. 

Year. 

a 

03 
1-5 

OS 

a 
< 

83 

d 

3 

D 

1-5 

si 

< 

D. 
0) 

o 

O 

c 
a 
< 

1871 

NE 

NW 
NW 

NW 

SW 
NW 
NW 
NW 
NW" 

W^ 
NW 
NW 

W" 
NW 

N 
NW 
NW 
NW 
SW" 

NW 

NW 

NW 

NW 

W 

w 
w^ 
w 

SW" 
SW 

W" 

w 

W" 

NW" 

NW 

w 

NW 

NE 
NW 
NW" 
NW" 
NW 

NW 

NW" 

W" 

W 

NW 

NW 
NW 
NW 
NW" 
NW 

NW 
NW" 
NW^ 
NW 
NE 

NW" 
NE 
NW 

N 

NW^ 

W 

w 

W" 
W" 
W" 

W" 

w^ 
w 

NAV 

NW 

W 

NW 

NW 
NW 

NW 

NW 
SE 

NW 

NW 
NW 
NW 

NW^ 

W 

NW^ 
NW" 
NW" 
NW 

NW 
NW" 
NW 
NW^ 

NW" 

NE 
NW 
NW 

NW 
NW 

NW 
E 
E 
E 
W 

W" 
W 

SE 

NW 

NW 
NW" 

NW 

W 

W" 

NW 

NE 
NW 

NW 

NE 
NW^ 
NW 

NW 

W 
NW 

N 
NW^ 
NW 

NE 
NW 
NW 

NE 

NE 

NW 
NW 

SE 
NW 

SE 

SE 
W 

w 

SE 
NW 

N 

W 

NW 

NW 
NW" 
NW 
NW 

NE 
NW^ 
NE 
SE 
SE 

SE 
NW" 
NW" 

SE 

S 

SE 

s 

SE 
NW 

NE 

NW 

SE 
SE 

SE 

S 

NE 

NW" 

W" 

SE 
SE 

SW 
NW^ 

SE 
SE 
W" 

E 
SW" 

SE 

SE 
SE 

SE 
SE 

SW 
SW 

SE 
SW 

s 

SE 
SE 
W" 
SW 
W" 

w 

s 
s 

SE 
NW^ 

SE 

S 

NW 

SW 

s 

NW 

SW" 

E 
SW^ 

E 

SW 
NW 
SW 
N 
SW^ 

SB 
NW^ 
SE 

SW 

s 

SW 
SW 

NW 

SW 
SW 
SW 

S 

NW 
SW 
SW 

s 
s 

W" 

s 

SW 

NW" 

SE 

SW 

s 
s 

SW 

s 

SW 
SW 
SW 
SW 
NW 

SW 

w 

SR 
SW 
SW" 

SW 
SW" 

w 

SW 

s 

SW 
SW" 

SW 

w 

NE 

N 
NE 

SE 

SE 

NW 

W 

S 

N 
S 
N 
N 
SW 

NW 

N 

SW" 
SW 

s 

NW 
NW 

SE 
SE 
SW 

N 
NW 
SW 
NE 
AV 

S 
SW 

NE 

SE 

N 

NW^ 

SW 

N 

N 

N 
NE 
SW" 

NE 

SE 

E 

SE 

NW 

SE 

N 
NE 

S 
N 

S 

N 

N 
NE 
NE 

NW" 
SE 
W 

NE 

N 

SW 

N 
SW 

SE 
SE 

N 
NW" 
NW^ 

N 

N 
SE 
N 

NW^ 

NW 
NW^ 

NW 
NW" 

NW^ 
NW 
NW" 

SE 
SE 

S 
NE 

N 
N 

N 

NW 

NW 
NW" 

NE 
NW 

NW 
NW 
NW 
NW 

N 

N 

N 

SE 

E 

E 

SE 
NW 
NW 

NW 
NW" 
NW 
NAV 

NW 

NAV 
NAA" 

NAV 
N 

NAV 
NAV 
W^ 
NW 
NW 

NW 

NE 

N 

NW 

NW 

NAV 
NAV 
NAV 
NAV 
NAV 

NAV 

NAV 

N 
NW 

N 

SW 

AV 

AV 

NAV 
NAV 

W 

N 
NW 

NAV 

NW 
NAV 
NAV 

SAV 
AA" 
NAV 
NAV 
NE 

NW 
SAV 
W^ 
E 

NAV 

AV 
NAV 
NAV 

N 
NW^ 

NAV 
NW 
NW 

NE 
NAV 

NAV 
NW 
SAV 

NW 
N 

W 
NW 
AV 
AV 
AV 

NAV 
NAV 
NW 

NAV 
NAV 
NAV 
NAV 

NW 

1873 

NW 

1873      

1874 

NAV 
NAV 

1875 

NW 

1876 

NAV 

1877 

1878 

1879 

NAV 
NAV 
NAV 

1880 

NAA" 

1881 

AV 

1883  

NAV 

1883 

NAV 

1884 

NAV 

1885 

NW 

1886 

1887 

NAV 

NAV 

1888  

1889 

NAV 
NW 

1890 

NW 

1891 

NAV 

1893 

NAV 

1893 

NAV 

1894 

NAV 

1895 

N 

1896 

SAV 

1897 

AV 

1898 

AV 

1899 

1900 

SE 
AV 

1901 

AV 

1903 

AV 

1903 

1871-1880 

NAV 
NAV 

1881-1890 

NW 

1891  1900 

NAV 

1871-1903  

NAV 

January,  1871-Oct6ber,  1879,  from  eye  observations  at  7.30  a.  m.,  4.30  p.  m.,  and  11.00  p.  m. 
November,  1879-December,  1886,     "       "  "  "   7.00     "      3.00     "        "     11.00    " 

January,      1887-June,  1888,      "        "  "  "    7.00     "      3.00     "         "     10.00    " 

July,  1888-November,  1893,     "       "  "  "   8.00     "     and  8.00  p.  m. 

December,  1893-December,  1903,     "      hourly  record. 

spring,  the  south  winds;  in  the  summer,  the  southwest;  in  autumn,  the 
winds  from  any  quarter  between  east  and  southwest  have  about  the  same 
temperature.  The  relative  position  of  the  winds,  arranged  according  to 
temperature,  Avith  the  Avarmest  first,  is  indicated  below: 


MARTLAXD    WEATHER    SERVICE 
RELATIVE  TEMPERATURE  OF  THE  WINDS. 


273 


Warmest. 

Coldest 

January 

SE 

E 

sw 

NE 

W 

s 

NW 

N 

April 

S 

SW 

SE 

w 

N 

E 

NE 

NW 

July 

sw 

S 

w 

SE 

NW 

NE 

E 

N 

October 

SE 

S 

sw 

E 

NE 

NW 

W 

N 

Tear 

SE 

sw 

s 

E 

W 

NE 

NW 

N 

COMPARATIVE  PREVALENCE  OF  STATED  DIRECTIONS. 
(In  average  number  of  hours  and  minutes  per  day.) 


NE 


January 3.54 

April 1.24 

July          2-3« 

October 4.18 


Average '2.54     1.24     3  24 


1.42 
1.54 
0.43 
1.42 


E 

SE 

S 

SW 

W 

3.06 
2.24 
1.13 
2.36 

2.36 
7.24 
3.18 
3.24 

0.30 
0.30 
3.12 
1.24 

3.06 
3.24 
7.12 
3.06 

5.30 
3.24 
3.24 
2.54 

3  24 

4.06 

1.13 

4.06 

3.48 

NW 


4.36 
3.36 
8.24 
4.36 


4.06 


In  the  above  table  the  figures  show  the  number  of  hours  and  minutes 
during  which  the  stated  winds  prevailed  during  the  five  years  from 
1893  to  1897.  For  example,  in  January  a  north  wind  prevailed  on  the 
average  for  the  five  years,  during  12  per  cent  of  each  day,  or  a  little  less 
than  three  hours.  This  is  equivalent  to  about  3.7  days  for  the  entire 
month.  A  south  wind  is  in  all  months  of  the  3'-ear  of  decidedly  shortest 
duration  and  of  least  frequency. 


Monthly  Frequency  of  Stated  Directions. 

A  four  years'  record  of  hourly  wind  directions  was  examined  and  the 
observations  tabulated  in  such  manner  as  to  show  the  number  of  days 
per  month  upon  which  the  wind  blew  from  each  quarter.  The  monthly 
number  of  days  for  the  entire  year  is  as  follows : 


.\  viTHgc  no.  of  days  . . 
Highest  number  


NE  j     E 

I 


SE 


19.0     16.0     15.9  I  16.4 
21.8     19.5     19.2     22.0 


IX)weBt  number 16.2     14.0     14.0  i    9.5 

1  I 


s 

SW 

W 

15.8 

18.7 

19.7 

19.5 

23.8 

22.8 

9.3 

11.0 

16.5 

NW 

20.5 
15.8 


274  THE    CLIMATE    OF    BALTIMORE 

The  above  figures  indicate  that  the  wind  blows  from  nearly  every 
quarter  once  in  about  two  days.  Take  for  example  a  northwest  wind; 
on  the  average  it  blows  on  20.5  days  per  month  the  year  round;  in  Janu- 
ary, February,  March  and  July  it  has  an  average  frequency  of  22.2  days, 
and  in  September  15.8  days.  We  may  also  learn  from  these  figures  that 
the  wind  blows  from  four  to  five  different  directions  every  day,  on  the 
average.  The  exact  figures,  based  on  the  four  years'  record,  are  as  fol- 
lows for  each  month  of  the  year : 

AVERAGE  DAILY  NUMBER  OF  WIND  DIRECTIONS. 


Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July  Aug-. 

Sept.  Oct. 

Nov. 

Dec. 

Year 

4.6 

4.3 

4.4 

4.8 

4.8 

5.2 

4.9       5.3 

4.6       4.1 

4.4 

1 

4.6 

4.7 

These  figures  are  in  harmony  with  the  facts  recorded  in  the  discussion 
of  the  diurnal  periodicity  of  wind  direction.  It  was  there  shown  that 
the  wind  backed  daily  from  a  westerly  direction  in  the  morning  to  south- 
east or  east  in  the  afternoon,  and  then  returned  again  at  night  to  the 
west  or  northwest ;  in  other  words,  that  the  wind  shifted  through  four  or 
five  points  by  noon  and  returned  to  its  original  position  at  night. 

The  Direction  of  Upper  and  Lower  Clouds. 

Table  LXXVI  is  inserted  at  this  point  simply  to  show  the  prevailing 
direction  of  the  wind  at  the  level  of  the  upper  and  lower  clouds.  The 
observations  cover  a  period  of  five  years  and  indicate  the  directions  at 
7  a.  m.  and  3  p.  m.  The  upper  clouds  include  all  cirrus  forms,  the  alto- 
stratus  and  alto-cumulus;  the  lower  forms  include  the  cumulus  and 
stratus  forms. 

The  upper  clouds  move  from  the  west  throughout  the  year,  both  in 
the  morning  and  afternoon,  with  an  occasional  exception  in  the  way  of  a 
northwest  or  southwest  direction,  especially  at  the  afternoon  observation. 
The  lower  clouds  are  also  mostly  from  the  west  at  7  a.  m.  from  May  to 
December;  from  January  to  April  they  are  generally  northwest.     In  the 


^klARYLAXD    WEATHER    SERVICE 


275 


TABLEILXXVU.— PREVAILING  DIRECTION  OF  LOWER  AND  UPPER  CLOUDS. 


1 

^ 

1 

'C 

1 

2 

-. 

bl 

X 

1 

z 

-< 

7  o.  m.... 

tr 

ir 

jr 

ir 

ir 

tr 

ir 

i««3if^:m:::: 

.... 

tr 
w 

tr 

.sir 
w 

ir 
sw 

tr 

X 

ir 
w 

.  3  p.  m.... 

\\" 

\\ 

\\ 

w 

w 

X 

w 

7  a.  m.... 

xir 

Air 

tr 

.vir 

tr 

tr 

ir 

ir 

A' 

ir 

ir 

3  p.  HI.... 

fr 

s 

ir 

xtr 

tr 

E 

tr 

If 

ir 

ir 

ir 
Air 

ir 

ir 

1884  - 

7  a-  m.... 

N 

NE 

X  w 

NW 

w 

NK 
SH 

\\ 

w 

w 

w 

NW 

w 

w 

3  p.m.... 

>> 

s 
s\\ 

w 

xw 

xw 

E 

w 

w 

X  w 

xw 

xw 

xw 

xw 

7«.  HI.... 

fr 

.VK 

ir 

ir 

.sir 
Air 

tr 

ir 
Air 

.sir 
tr 

ir 

.s'lr 

ir 

Air 

ir 

3p.  m.... 
1885-1 

Ta.  m.... 

}r 

(r 

ir 

xtr 

sir 
tr 

E 

tr 

tr 

II' 

tr 

ir 

Air 

ir 

ir 

sw 
w 

N 

\\ 

\\ 

xw 

\\ 

w 

\\ 

sw 

w 

xw 

w 

[sp.m.... 

w 

NW 

w 

sw 
xw 

xw 

xw 

xw 

sw 

\\ 

sw 

NW 

xw 

xw 

7a.  HI.... 

»r 

STF 

Nir 

tr 

ir 

A-ir 

Air 

tr 

.s'  tr 
xtr 

ir 

ir 

»r 

$p.  in..  . 

Tr 

Sir 

Nir 

NW 

xtr 

tr 

Air 

tr 

.sir 
Air 

XE 

tr 

ir 

ir 

ir 

1886 

7a.  m.... 

w 

NW 

NW 

NW 

NE 
NW 

NE 

w 

w 

w 

w 

X 

w 

sw 
xw 

w 

3  p.m.... 

sw 
w 

xw 

w 

XW 

NW 

X  \v 

xw 

sw 
w 

w 

xw 

xw 

\v 

NW 

7a.  HI.... 

ir 

IC 

ir 

tr 

ir 

ir 

tr 

tr 

tr 

ir 

ir 

ir 

ir 

3 p.  HI.... 

w 

ir 

ir 

X 

tr 

ir 

ir 

tr 

tr 

tr 

ir 

ir 

tr 

ir 

1887  ^ 

7  a.  m.... 

sw 

SE 

sw 

xw 

XE 
NW 

s 

XE 

sw 

NE 

sw 

w 

w 
sw 

w 

sw 

3  p.m.... 

NW 

NE 
NW 

NW 

SE 
NW 

E 

w 

sw 

NW 

N 

SE 

xw 

NW 

xw 

xw 

j   7  a.  »!.... 

w 

ir 

Air 

)(' 

tr 

tr 

IK 

1888  -{  3 p.  m.... 
1  7  a.  m.... 

NW 

w 

ir 
w 

IC 

w 

tr 
sw 

tr 
sw 

.... 

W 
W 

13  p.m.... 

w 

w 

NW 

X  \v 

sw 

\v 

W 

1   7a.  HI,... 

w 

ir 

((• 

tr 

tr 

tr 

tr 

tr 

tr 

tr 

ir 

ir 

ir 

1    3  IJ.  III.... 

jr 

tr 

tr 

xtr 

tr 

ir 

tr 

ir 

tr 

tr 

tr 

ir 

ir 

Prevail,  i  7  a.  m.... 

sw 

NW 

N  E 

xw 

NW 

\\ 

w 

w 

w 

w 

w 

w 

w 

w 

3  p.m.... 

w 

NW 

w 

NW 

xw 

w 

NW 

N  w 

\v 

w 

xw 

X  w 

NW 

NW 

a.      ] 

a.  s.    1 

Vpiiff  chpiiih.  ■{  Ci.  Cit.  y  in  Italic. 
I   .1.  Ci(.   1 
L  A.  S.     J 


Lower  clouds.  •{   |-  ^'"• 


f  Cu. 

!    S.    ■ 

I  s 

L  N. 


in  Roman. 


afternoon,  a  northwest  direction  prevails  during  eight  months  of  the 
year  in  the  lower  cloud  layer,  with  a  direction  from  the  west  in  January, 
March,  August  and  September. 


19 


276 


THE    CLIMATE    OF    BALTIMORE 


ELECTEICAL   PHENOMENA. 

Thunderstorms. 

The  intimate  relation  existing  between  thunderstorm  formation  and 
temperature  is  demonstrated  by  an  inspection  of  Table  LXXVII  and 
Fig.  77.     The  maximum  frequency  occurs  in  the  month  of  greatest  heat 


TABLE  LXXVII.-HOURLY  FREQUENCY  OF  THUNDERSTORMS. 


Midn't  to  1  a.  m. 

1-  2 

2-3 

3-4 

4-  5 

5-  6 

6-  7 

7-8 

&-  9 

9-10 

10-11 

11-Noon.. 
Noon-  1  p.  m 

1-2 

2-3 

3-4 

4-  5 

5-  6 

6-  7 

7-8 

8-  9 

9-10 

10  11 

ll-Mldn't. 

Totals 


24 


97 


141 


164 


100 


38 


61 
66 
69 
38 
34 
31 
16 
13 


610 


Table  LXXVII  shows  the  total  number  of  thunderstorms  recorded  as  begin- 
ning within  the  stated  hours  in  the  27  years  from  1876  to  1903,  during  each 
month  and  during  the  entire  year. 

and  at  the  hour  of  the  daily  maximum  temperature.  In  tabulating  all 
thunderstorms  which  passed  over  Baltimore  during  a  period  of  28  years, 
a  total  of  678,  we  find  the  following  distribution  by  months: 


Jan. 

Feb. 

Mar. 

Apr. 

Maj'   June  July 

Aug. 

Sept.  Oct. 

Nov. 

Dec. 

Year 

13 

11 

20 

33 

107       1.56       179 

Ill 

43         7 

6 

2 

1 

678 

\ 


IMARYLAXD    WEATHER    SERVICE 


277 


- 

i       ' 

1 

0       1 

Na 

ON         1 

2 

3 

4 

5 

6 

7 

8 

9 

1 

)       1 

Mdt 

1  1 

' — ' 

s — i 

• 

t 

. 

^>-. 

N 

^^^-^/T\: 

•i^ 

x 

> Ir 

4 

^^ 

s 

, 

• 

• 

• 

" 

u 

V 

^ 

^ 

\! 

— 

LV 

'. 

• 

— 1 

• 

( 

^fe—V^y 

^\ 

— 

-^ 

•  "^ 

■ 

Fig.  77. — The  Frequency  and  Distribution  of  Thunderstorms. 

The  diagram  represents  over  650  thunderstorms  which  passed  over  Baltimore  in  the 
30  years  from  1871  to  1900.  The  density  of  the  shading  increases  with  the  frequency 
of  the  storms,  showing  a  maximum  frequency  between  3  p.  m.  and  5  p.  m.  in  the  month 
of  July.  For  the  hours  of  the  day  and  month  during  which  less  than  five  storms  were 
recorded  in  30  years,  the  actual  number  is  indicated  by  the  small  dots.  The  figures 
attached  to  the  curved  lines  represent  the  total  frequency  in  30  years. 


1 

1 

1 

1 

,ll 

1, 

1 

1     1 

1 

Fio.  78. — The  Average  Monthly  Frequency  of  Occurrence  of  Thunderstorms. 


278 


THE    CLIMATE    OF    BALTIMORE 


About  five-sixths  of  the  total  annual  number  occur  in  the  months  of 
May,  June,  July  and  August.  In  the  winter  months  they  occur  only  at 
rare  intervals,  generally  in  connection  with  cyclonic  storms  which  ex- 
hibit strong  contrasts  in  temperature.  The  summer  storms  occur  mostly 
in  connection  with  shallow  and  not  very  well  defined  cyclonic  depres- 


1880 

1885 

1890 

1895 

1900 

30 

20 

10 

0 

Fig.  79. — The    Annual    Frequency    of   Occurrence    of   Thunderstorms    from 
1871  to  1904. 

sions.     That  there  is  a  strongly  marked  diurnal  periodicity  in  the  for- 
mation of  the  summer  thunderstorms  is  shown  bv  the  following  figures: 


HOURLY  FREQUENCY  OF  THUNDERSTORMS. 


Hours  ending 

Morning' 

Afternoon 1  22 


3 

1  I    3 
45     76     78 


5       fi       7  8  9  10     n  12 

I 

0       3       6  fi  6  8       9  1.5 

61      .55     69  3d  ,  31  ,  31  .  16  13 


:XIARYLAXD    WEATHER    SERVICE 


279 


The  hourly  distribution  for  all  months,  expressed  in  terms  of  the 
total  frequency  in  30  years,  is  shown  in  Fig.  77.  The  monthly  and 
annual  distribution  by  years,  from  1876  to  1903,  is  shown  in  Table 
LXXVIII.     The  annual  changes  in  frequency  are  also  shown  graphically 


TABLE  LXXVIII.— NCMBER  OF  THUNDERSTORMS  PER  MONTH. 


Year. 


1876 .. 

187T 

1878 

1879 

1880 1 

1881 

1882 

1883 

1884 

1885 

1886 

1887 

1888 

1889 

1890 

I89I 

1892 

1893 

1894 

1895 

1896 

1897 

1898 1 

1899 1 

1900 

1901 .. 

1902 

1903 

Totals,  1876-1903...       3 
Averatre 0.1 


11 

0.4 


•?         — 


30 
0.7 


33 
1.2 


107 
3.8 


166 
5.6 


6 

15 

3 

6 

5 

5 

2 

5 

8 

7 

5 

1 

« 

4 

3 

3 

8 

5 

11 

7 

7 

6 

8 

7 

8 

6 

6 

7 

179 
6.4 


111 

4.0 


4H 
1.5 


26 
16 
14 
8 
20 

21 

18 
36 
20 

37 

19 
24 
17 
16 

18 

25 
31 
21 
43 

24 


25 


0.2      0.1 


678 
24.2 


in  Fig.  79.  The  average  annual  number  is  appro.\iinately  '■^4,  with  a 
maximum  frequency  of  42  in  189-4,  and  a  minimum  of  8  in  1879.  The 
following  figures  express  more  exactly  the  average  monthly  and  annual 
frequency  (See  also  Fig.  78)  : 


.lull. 

Fob. 

Mar. 
0.7 

A,,r. 

•May 

.June 

July 

Aug. 

Sept.!  Oct. 
1.6      0.2 

Nov. 
0.3 

Dec. 

Yiar 

0.1 

0.4 

1.2 

3.8 

6.6 

6.4 

4.0 

0.1. 

24.2 

280  the  climate  of  baltimore 

Thunderstorm  Probability. 

The  probability  of  the  occurrence  of  a  thunderstorm  upon  any  desig- 
nated day  may  be  expressed  in  a  very  rough  way  by  finding  how  many 
times  a  storm  occurred  on  that  day  in  the  past.  By  examining  all 
records  of  thunderstorms  for  27  years  and  arranging  them  according  to 
the  day  of  the  month  upon  which  they  occurred,  we  may  roughly  obtain 
a  percentage  of  probable  occurrence. 

Not  much  reliance  should  be  placed  upon  such  a  method  of  forecast- 
ing, but  some  interesting  relative  values  are  brought  out.  In  the  past 
27  years  one  or  more  thunderstorms  have  occurred  on  every  day  of  May, 
June  and  July,  and  on  all  but  one  day  in  August  (the  20th)  ;  no  thun- 
derstorm has  occurred  on  September  6,  13,  21  to  23,  27,  28  or  30.  In 
April  there  is  no  record  of  a  storm  on  the  following  days :  1,  3,  6,  7,  13 
to  15,  21  to  23,  25,  30.  The  highest  number  occurring  on  any  stated 
day  in  May  is  7,  on  the  21st;  in  June,  11  on  the  21st;  in  July,  11  on 
the  5th;  in  August,  7  on  the  12th.  Hence  the  highest  probability  of 
occurrence  upon  any  day  in  the  year  is  only  eleven  twenty-sevenths,  or 
about  41  per  cent.  The  average  probability  for  a  day  in  May  is  14  per 
cent;  in  June,  20  per  cent;  in  Jnlj,  23  per  cent;  in  August,  15  per  cent. 
The  probability  for  the  Fourth  of  July  is  only  17  per  cent,  or  5  per  cent 
less  than  the  average  for  July  days.  According  to  the  Baltimore  records 
a  thunderstorm  has  passed  over  the  city  on  July  4  only  five  times  in  29 
years.  One  has  occurred  11  times  on  July  5,  in  the  same  period,  making 
the  maximum  probability  41  per  cent  of  the  total  number  of  such  days 
in  29  years. 

Consecutive  Days  with  Thunderstorms. 

Thunderstorms  generally  occur  as  isolated  storms  in  the  vicinity  of 
Baltimore.  In  over  80  per  cent  of  all  instances,  a  second  storm  does 
not  occur  on  the  following  day.  In  only  14  per  cent  of  all  cases  have 
there  been  thunderstorms  recorded  on  two  successive  days,  and  in  only 
a  little  over  three  per  cent  have  storms  occurred  on  three  successive 
days.  Only  on  one  occasion  have  as  many  as  5  occurred  on  5  successive 
days.     These  percentages   vary   in   different  months   but  they   are   not 


MARYLAND    WEATHER   SERVICE 


281 


large  in  any  month.      The  following  table  shows  the  figures  for  each 
month  and  for  the  year: 

CONSECUTIVE  DAYS  WITH  THUNDERSTORMS. 
(Total  number  in  28  years.) 


s 

1-5 

si 

p. 
< 

o 
a 

3 

si 
< 

OQ 

O 

> 

o 

03 

3 

11 

16 

i 

26 

2 

0 

65 
13 
2 

87 
18 
4 
1 
1 

94 

21 

6 

3 

6T 

12 

4 

26 
6 

1 

T 

6 

2 

410 

72 
17 

4 

1 

On  2  consecutive  days  

"3           "               "    

"4           "               "    

"5           "                "     

s.  w. 
Fic;.  80. — The  Direction  of  Movement  of  Thunderstorms. 

The  diagram   shows  the  actual  and  relative  frequency  of  thunderstorms  from  each 
direction  of  the  compass.     The  total  number  of  storms  represented  is  nearly  400. 

Direction  of  Thunderstorms. 

In  the  vicinity  of  Baltimore,  thunderstorms  usually  come  into  view 
from  some  point  between  northwest  and  southwest.     Out  of  a  total  of 


282 


THE    CLIMATE    OF   BALTIMORE 


about  400  storms,  nearly  90  per  cent  moved  from  some  one  of  tliese 
points.  (See  Fig.  80.)  The  order  of  frequency  of  direction  is  as 
follows : 

THUNDERSTORM  DIRECTIONS. 
(1876-1902.) 


Jan. 

Feb. 

Mar. 

Apr. 

8 

3 

I 

May 

June 

July 

Aug. 

21 
18 
21 

3 

2 

2 

'3 

Sept. 

18 
6 

'i 
i 

Oct. 

1 
2 

2 

Nov. 

"i 

Dec.  Year 

NW  to  SE 

1 
i 

3 

3 
3 
4 

i 

26 

19 

13 

2 

2 

2 

20 
29 
31 

'3 

1 

1 

33 

32 

20 

3 

6 

'5 
3 

..       126 

\V       •*   E        

..       115 

SW    "    NE 

S        ••    N      

..       104 

s 

SE     "    XW 

E       "    W     

NE    "    SW 

1         14 
4 

N       "    S      

13 

Pressure  Changes  Duking  Thunderstorms. 

A  thunderstorm  usually  occurs  with  a  falling  barometer;  the  bar- 
ometer rises  during  the  first  few  minutes  after  the  storm  has  begun, 
falls  slightly  before  the  close  of  the  first  hour,  and  then  maintains  a 
steady  pressure  for  several  hours,  eventually  rising  slowly  (see  Pig.  81). 
In  other  words,  the  storm  usually  breaks  in  the  trough  of  a  cyclonic  dis- 
turbance.    The    following   table   shows   the    average    hourly    barograpli 

PRESSURE  BEFORE  AND  AFTER  THUNDERSTORMS. 
(Station  readings;  not  reduced  to  sea-level.) 


No. 

Hours  Preceding. 

Rise. 

Hours  following. 

Month. 

5th 

4th 

3rd 

3nd 

1st 

Begin- 

P. 

'H 

1st 

2nd 

3rd 

4th 

5th 

29.45 

ning. 

H 

ss 

.30 

.20 

Feb.... 

(3) 

.43 

.40 

.36 

.34 

29.31 

0.25 

29.36 

.34 

.26 

.18 

March. 

(3) 

.67 

.63 

.60 

.56 

.51 

.49 

0.25 

.53 

.51 

.51 

.51  !     .53 

..>t 

April.. 

(2) 

.38 

.38 

.38 

.38 

.38 

.38 

MS 

.47 

.44 

.44 

.43 

.44 

.44 

May... 

118) 

.78 

.77 

.76 

.76 

.74 

.74 

0.42 

.79 

.77 

.77 

.78 

.79 

.80 

June... 

(16) 

.79 

.78 

.77 

.76 

.75 

.74 

0.43 

.77 

.75 

.74 

.74 

.75 

.75 

July... 

(19) 

.77 

.77 

.75 

.74 

.73 

.72 

0.51 

.78 

.74 

.74 

.74 

.74  1 

.76 

Aug... 

(11) 

.80 

.80 

.79 

.79 

.79 

.79 

0-40 

.84 

.81 

.80 

.79 

.79 

.80 

Sept. . . 

(6) 

.76 

.75 

.74 

.73 

.72 

.70 

0.53 

• 

.76 

.74 

.75 

.77 

.78 

.80 

Aver... 

(76) 

.68 

.66 

.65 

.63 

.62 

.61 

.45 

.66 

.64 

.63 

.63 

.63 

.6?$ 

*  Time  from  beginning  of  the  rise  to  its  maximum,  in  hours  and  minutes. 

readings  before  and   after  about  75   thunderstorms  selected  from  the 
records  of  the  past  three  or  four  years.     The  readings  of  the  barograph 


3»ia  ' 


M«.rck  3o,  130I 


M».rt.lv  2},  ?9e3. 


flpr.  2,6,  I9»2.. 


Moy  i,  <»o2.. 


Muy  iS.  1902. 


F«.l).a8,  (>«3 


/Mav,  i*.  f9»3. 


iM»3  (1,  I9»  +  . 


Ju  N   3,  1901. 


7u^y  20,  '^oi. 


7u/y    ',  'J't. 


flgg    fc,  i9o2 


/?uj.    i  7,  (  j«z 


ffw)   2  9  j9oS. 


Fio.  81. — Some  Typical   Barograms   During  Thunderstorms  and   Squalls. 


284  THE    CLIMATE    OF   BALTIMORE 

are  given  for  the  five  hours  preceding  and  following  the  breaking  out  of 
the  storm.  The  minimum  reading  is  also  given,  just  before  the  begin- 
ning of  the  "  hump/'  which  constitutes  the  characteristic  feature  of  a 
barograph  curve  during  the  passage  of  a  thunderstorm.  The  duration 
of  the  rise  in  pressure,  from  the  minimum  to  the  maximum  point  attained 
in  the  "  hump  "  is  given  in  hours  and  minutes. 

Of  the  76  thunderstorms  examined  in  the  above  table,  about  one-third 
began  with  a  value  between  29.60  inches  and  29.69  inches  for  the  bar- 
ometer reading,  assuming  the  beginning  of  the  rise  in  tlie  barometer  to 
be  the  beginning  of  the  storm.  Tabulating  the  barometer  readings  ac- 
cording to  the  pressure  at  the  breaking  out  of  the  storm,  we  have  the 
following  comparative  frequency  of  stated  values : 

FREQUENCY  OF  STATED  READINGS  OF  THE  BAROMETER  AT  THE  BEGINNING 

OF  THE   STORM. 


Barometer  Re 

29.30-39  in 
4049 

ading. 

;hes 

Actua 

3 

5 

Frequency 
I.              Pel 

•centage. 
4 

7 

50-59 

5 

7 

60-69 
70-79 
80-89 
90-99 

24 

14 

17 

7 

32 

18 

22 

9 

30.00-09 

1 

1 

7G  100 

The  thunderstorms  in  the  above  table  were  confined  almost  entirely  to 
the  months  of  May  to  August.  We  see  that  the  storm  broke  most  fre- 
quently when  the  pressure  registered  some  value  between  29.60  inches 
and  29.69  inches;  this  was  true  of  32  per  cent  of  all  cases  tabulated; 
in  72  per  cent  of  all  cases  the  barometer  reading  was  between  29.60 
inches  and  29.89  inches.  In  only  one  instance  was  the  pressure  above 
30.00  inches,  namely,  in  July,  1900.  The  lowest  pressure  recorded  in 
any  case  was  29.30  inches,  in  February,  1903.  See  Fig.  81  for  the 
character  of  the  rise  in  pressure  during  thunderstorms. 

Hail. 

The  phenomenon  of  hail  formation  is  so  intimately  associated  with  the 
dynamics  of  thunderstorms  that  the  treatment  of  the  subject  is  taken 


MARYLAND    WEATHER   SERVICE 


285 


up  in  connection  with  these  storms  rather  than  with  the  subject  of  pre- 
cipitation. Hail  is  not  of  frequent  occurrence  in  the  vicinity  of  Balti- 
more. During  a  period  of  28  years  it  has  been  recorded  but  49  times, 
or  less  than  two  times  per  year.  The  annual  number  has  varied  between 
0  and  a  maximum  of  6.  The  monthly  and  annual  distribution  is  shown 
in  Table  LXXIX,  and  the  hourly  distribution  by  months  in  Fig.  82. 


TABLE  LXXIX.-FREQUBNCY  OF  OCCURRENCE  OF  HAIL. 


Year. 


1876. 
1877. 
1878. 
1879. 
1880. 

1881 

1883. 
1883. 
1884. 
1885. 


188fi. 

1887.. 
1888.. 
1889.. 
1890.. 


1891. 
1892. 
189:5. 
1894. 
1896. 

1896. 


1899. 
1900. 

1901. 

1903. 


Total  in  28  years... 


9  I  6 


66 


Tlie  liourly  distribution  for  the  entire  year  is  as  follows: 


HOURLY  FREQUENCY  OF   HAIL. 
(Total  number  iu  28  years.) 


Time  A.  M.  1    9 

10 

11 

Noon. 

1 

Z 

8      4       6 

6 

7 

8 

9 

10 

11 

P.M. 

Frequency,  i    2 

1 

1 

2 

8 

<       1        i 
0       4       6     13 

7 

4 

8 

2 

0 

1 

286 


THE    CLIMATE    OF    BALTIMORE 


5  6  7  8  9         10        II      Noon     12  3  4  5 


7  8  9  10        tl       MO 


■ 

1 — 

' 

....i. 

.it' 

(1    - 

1  ■ 

• 

•[ 

Fig.  82. — The    Frequency    of    Occurrence    and    the    Hourly    and    Seasonal 
Distribution  of  Hailstorms. 

The  diagram  show.s  all  of  the  hailstorms  recorded  as  occurring  in   Baltimore  during 
a  period  of  28  years.     Each  black  dot  represents  a  storm. 


Mdt.  Noon  Mot  Noon  Mot.  Noon  Mdt 


Inches 

1 

2 

3 

30.00 

^ 

-^ 

.50 
29.00 
30  00 

s        ~--— 

4 

5 

6 

29.50 

—     ■ 

-..^ — 

Fig.  83. — Barograms  During  Hailstorms. 

Each  barogram  represents  a  period  of  24  hours,  from  midnight  to  midnight.  The  time 
of  occurrence  of  the  hail-storm  is  indicated  by  the  sharp  temporary  rise  and  fall  in 
the  curve. 

Dates  of  the  storms  represented  :  1.  .July  7,  lOiH.  2.  February  28,  1002.  .3.  August 
27.  1902.     4.   June  8.  190.3.      .").   May   It).   1904.      6.   .July  5.   1004. 


MARYLAND    AVEATHEU    SERVICE 


287 


The  hourly  frequency  rises  to  a  maximum  between  4  p.  m.  and  5  p.  m. 
The  dates  of  all  recorded  occurrences  of  hail  in  the  vicinity  of  the  Balti- 
more station  of  the  Weather  Bureau  are  siven  in  Table  LXXX. 


TABLE  LXXX.— DATE  AND  HOUR  OF  OCCURRENCE  OF  HAIL. 


Date 

Time 

- 

First 
precip.* 

Date 

Time 

First 
precip. 

1876,  Mar.  2» 

7.1.5  p.  ra. 

1.S95,  July    5 

12.25  p.  m.-12.30  p.  m. 

"     May   12 

2.1.5  p.  m.-  2.30  p. 

ra. 

"       16 

+          

4.'36p 

"      21 

9.35P 

"     Aug.  11 
"        "      31 

4. .50  p.  m.-  4.52  p.  m. 
4..30p.  m.-  4.35  p.m. 

1879,  June  11 

2.00  p.  m.-  3.03  p. 

m. 

.,     28 

+          

4.'29p 

"     Sept.  19 

+          .... 

3.10p 

1880,  Apr.  17 

Early  a.  m. 

1S96,  July  27 

8  13  p.  ra.-  8.17  p.m. 

"     July  20 

t 

4.40p 

1897,  Mav   21 
"     Aug.  23 

1.45  p.m.-  1.55  p.m. 
11.47  a.  m.-11.52a.  m 

1881,  June   8 

1882,  ••     19 

t 

3.15p 
Noon. 

1898,  May   16 

\   4.10  p.m.-  4.13  p.m. 
1    5.14  p.  m.-  5.17  p.  m. 

1884,  July  11 

4.00  p.  m.-  4. .50  p. 

ra. 

1899,  Mar.  12 

J    8.27  p.  m.-  8  35  p.  m. 

1887,  Feb.  18 

5.02  p.  m.-  5.05  p. 

ra. 

•'      28 

8.07  a.  m.-  8.09  a.  ra. 

"     May  26 

t 

5.26p 

'•     Apr.  16 

jl0.25a.ra.-     .... 
1 12.20  p.  m.-     .... 

"    June  18 

+          

4.35p 

"    July  18 

5.10p 

"     May   16 

t 

6.50p 

1888,  June  16 

2.30  pVin.-  2.35  p. 

m. 

"     June  6 

t 

7.22p 

"     23 

t 

ll.'o.^a 

"     Aug.  21 

7. '5:3  p.  m.-  7.3S  p.  m. 

"     Aug.    8 

5.45  p.  m.-  5. .50  p. 

m. 

1901,  May   25 

t 

10  15p 

1890,  Apr.  27 

3.45  p.  m.-  4.00  p 

m. 

"     July    7 

7.00  p.  m 

"     May   U 

t          

e'.srlp 

1902,  Feb.  2.s 

9..50a.m 

"    June  12 

3.55p 

"     Aug.  27 

4.43  p.  m.-  5.05  p.  ra. 

1892,  Mav  2:! 

i  4.08  p.  m.-  4.]6p 

m. 

19C3,  May  24 

3.24  p.  ra.-  3.27  p.  m. 

"     June  30 

12. 02  p.  m.-12.10p 

m. 

"    June   8 

3.27  p.  m.-  3.33  p.  m. 

1893,  July    3 

5.25  p.  m.-  6.35  p 

m. 

J, 

t 

4."l6p 

1894,  May     6 

fi..57  p.  m.-  7.03  p 

m. 

"    June  12 

4.37  p.  m-  4.40  p 

m. 

"      24 

t 

4.'l6p 

*  1  n  the  absence  of  the  exact  time  of  occurrence  of  hail  the  time  of  beginning  ot  precipi- 
tation is  given. 
+  Hail  in  city  or  suburbs ;  none  at  station. 
i  Not  accompanied  by  a  thunderstorm. 

In  Fig.  83  a  few  typical  barograms  are  reproduced  showing  the  char- 
acteristically sharp  rise  and  fall  of  the  atmospheric  pressure  during  the 
passage  of  a  hailstorm.  In  the  thunderstorm  curve  the  summit  of  the 
"  hump  ''  is  usually  more  rounded,  as  shown  in  Fig.  81. 


288 


THE    CLIMATE    OF    BALTIMORE 


Auroras. 

The  following  brief  list  contains  all  occurrences  of  the  aurora  borealis 
reported  in  the  records  of  the  U.  S.  Weather  Bureau  at  Baltimore  since 
the  establishment  of  the  station  in  1871 : 


Date. 

Duration. 

Date. 

Duration. 

1872,  Feb.    3 

8  p.  m.  to   9  p.  m. 

1892,  Feb.   13 

6.30  p.  m.  to  9  p.  m. 

Apr.  11 

8  p.  m.  "  10  p.  m. 

May   18 

8  p.  m.  "  11  p.  m. 

Auf?.   3 

8.40  p.  m.  "  10  p.  m. 

July  16 

10.30  p.  m.  "  11.30  p.  m. 

Aug.    4 

About  9  p.  m. 

1893,  Feb.     4 

9  p.  m.  "  12  md't. 

Aug.    8-9 

9  p.  m.  to   3  a.  ra. 

1894,  Feb.   23 

9  p.  m.  "  10  p.  m. 

Oct.    14 

6.30  p.  m.  "     7  p.  m. 

Mar.  30 

7.20  p.  ra.  "  early  a.  m. 

Nov.    1 

10  p.  m. ."  11  p.  m. 

1897,  Jan.    23 

Evening. 

1873,  June  36 

About  10  p.  m. 

1898,  Sept.    2 

About  10  p.  ra. 

1882,  Apr.  16-17 

10  p.  m.  to   3  a.  m. 

1903.  Oct.    12 

7  p.  m.  to  7.30  p.  m. 

Apr.  20 

12.30  a.  m.  to   3  a.  m. 

SuxspoTS  AND  Weather. 

The  effort  to  extend  the  period  covered  by  weather  forecasts  has  ever 
been  one  of  the  chief  aims  of  the  practical  meteorologist.  The  limit  of 
time  for  which  forecasts  are  now  issued  by  American  and  European 
official  weather  services  is  about  three  days.  The  forecasts  made  from 
day  to  day  generally  cover  from  24  to  48  hours ;  under  favorable  conditions 
the  time  is  occasionally  extended  to  three,  or  even  four  days,  but  this 
is  only  done  in  exceptional  cases.  The  three  or  four  day  limit  is  probably 
the  utmost  that  will  be  realized  from  present  methods,  and  with  the 
material  now  at  our  disposal.  The  only  hope  of  extending  the  period 
lies  in  the  discovery  of  some  new  laws  of  weather  sequences. 

The  search  for  periodical  recurrences  of  similar  weather  conditions  has 
long  been  one  of  the  most  interesting,  and,  at  the  same  time,  one  of  the 
most  elusive  problems  in  cosmical  ph3^sics.  The  investigations  have 
usually  been  along  two  lines :  A  series  of  observations  has  been  subjected 
to  close  examination  and  critical  analysis  in  order  to  discover  any  periodic 
change  which  may  be  hidden  in  the  constant  fluctuation  of  values;  or  a 
periodic  movement  has  been  assumed  and  the  weather  observations 
examined  for  synchronous  changes. 

There  is  but  one  undisputed  source  of  terrestrial  weather  changes — ; 
namely,  the  sun.     While  no  one  doubts  the  influence  of  the  sun  upon 


MARYLAND   WEATHER    SERVICE  389 

the  earth^s  atmosphere  many  claims  have  been,  and  are  still  being  made 
in  favor  of  attributing  to  other  heavenly  bodies,  such  as  the  moon  or  the 
planets,  a  considerable  effect.  The  champions  of  the  moon's  influence 
are  legion,  and  they  never  grow  less;  but  the  arguments  of  several 
centuries,  including  much  serious  and  intelligent  effort,  have  not  suc- 
ceeded in  securing  for  lunar  or  planetary  forecasts  a  position  more 
exalted  than  the  pages  of  the  perennial  almanac. 

It  is  now  approximately  100  years  since  a  definite  period  was  dis- 
covered in  the  increase  and  decrease  of  sunspot  frequency,  and  less 
than  50  years  since  the  flames  emanating  from  the  surface  of  the 
Sim,  or  the  solar  prominences,  were  first  observed.  The  first  definite 
relation  between  sunspot  frequency  and  terrestrial  changes  was  the 
discovery  of  the  synchronous  activity  of  the  magnetic  needle.  There 
is  now  no  question  about  the  coincidence  of  these  phenomena  whatever 
may  be  the  true  relation  existing  between  them. 

In  attributing  terrestrial  changes  of  the  weather  to  "  sunshine,"  we  have 
until  comparatively  recent  times  assumed  a  constant  output  of  radiant 
energy  from  the  sun.  In  view  of  the  fact  that  our  present  knowledge 
concerning  the  physical  condition  of  the  sun  indicates  a  surrounding 
atmosphere  composed  of  incandescent  metallic  vapors,  is  it  not  more 
rational  to  suppose  that  the  temperature  of  these  highly  heated  gases 
is  var\ang  constantly,  than  it  is  to  think  of  them  as  at  a  constant 
temperature?  If  the  temperature  does  vary,  the  fluctuations  must 
necessarily  affect  to  a  greater  or  less  degree  the  physical  condition  of 
our  own  atmosphere.  The  question  then  becomes  one  of  degree  of 
influence.  There  are  many  observed  facts  which  point  to  a  varying 
output  of  solar  radiant  energy,  and  quantitative  measurements  will  not 
long  remain  unknown.  Just  what  the  nature  of  this  influence  is  has 
certainly  not  yet  been  demonstrated.  One  obstacle  in  the  way  of  more 
rapid  progress  toward  a  solution  of  these  problems  may  be  found  in  the 
crudeness  of  much  of  our  observational  data,  and  the  lack  of  uniformity 
in  the  methods  and  hours  of  observation.  Moreover,  in  the  middle  lati- 
tudes where  most  of  our  best  observations,  and  the  longest  scries  which 
we  possess,  have  been  made,  the  non-periodic  fluctuations  are  so  much 


390  THE    CLIZilATE   OF    BALTIMORE 

greater  than  the  periodic  changes  sought  that  the  latter  are  separated 
out  from  the  former  only  with  the  greatest  difficulty  and  care.  The 
most  favorable  regions  of  investigation  for  periodicities  based  on  solar 
changes  are  the  tropics.  Here  the  daily,  seasonal,  and  incidental  changes 
in  weather  conditions  are  more  uniform  and  less  pronounced,  permitting 
of  more  ready  detection  of  the  periodic  changes  of  longer  duration. 

It  seems  highly  probable  that  changes  in  terrestrial  temperature,  in 
rainfall,  storm  frequency,  etc.,  may  be  due  to  changes  in  the  physical 
constitution  of  the  sun's  surface.  It  may  also  be  that  these  solar  changes 
are  not  reflected  directly  in  the  conditions  above  mentioned.  Similar 
weather  conditions  should  not  be  expected  in  all  parts  of  the  earth  at 
the  same  time.  The  results  of  efforts  thus  far  to  find  a  direct  connection 
between  the  sunspots  and  weather  changes  have  apparently  failed  largely 
as  a  consequence  of  dissimilar  weather  conditions  found  in  different 
localities  during  similar  phases  of  the  solar  period.  These  contra- 
dictions may  be  only  apparent,  not  real.  Let  us  suppose,  for  instance, 
that  the  normal  distribution  of  pressure  over  large  areas  is  disturbed 
as  a  result  of  changes  in  the  quantity  of  heat  received  from  the  sun 
from  year  to  year.  We  would  then  have  excessive  heat  in  some  places 
and  at  the  same  time  abnormal  cold  at  others;  or  we  would  have 
excessive  rains  here  and  droughts  there;  or  an  increase  in  storm  fre- 
quency in  one  place  and  a  decrease  in  another,  when  compared  with 
average  conditions.  Such  variations  cannot  be  looked  upon  as  contra- 
dictory; they  are  the  natural  results  of  changes  in  the  distribution  of 
pressure,  changes  such  as  we  see  upon  our  weather  charts  every  day. 

The  present  status  of  the  problems  concerning  the  relation  between 
the  varying  physical  conditions  of  the  sun  and  synchronous  changes  in 
our  terrestrial  atmosphere  is  well  stated  in  the  following  extract  from 
a  recent  paper  by  Professor  Bigelow,*  who  is  one  of  the  most  active  and 
able  investigators  in  this  most  promising  field  of  cosmical  physics. 

"  The  numerous  studies  during  the  past  fifty  years  into  the  apparent 

*Bigelow,  F.  H.  Synchronism  of  the  Variations  of  the  Solar  Prominences 
with  the  Terrestrial  Barometric  Pressures  and  the  Temperatures.  Monthly 
Weather  Review,  Washington,  D.  C.  November,  1903. 


MAETLAXD    WEATHER    SERVICE  291 

synchronism  between  the  solar  variations  of  energy  and  the  terrestrial 
effects,  as  shown  in  the  magnetic  field  and  the  meteorological  elements, 
have  been  on  the  whole  unsatisfactory,  if  not  disappointing.  Just  enough 
simultaneous  variation  has  been  detected  in  the  atmospheres  of  the  sun 
and  the  earth  to  fascinate  the  attentive  student,  if  not  to  justify  a  large 
expenditure  of  labor,  in  view  of  the  great  practical  advantages  to  be 
obtained  in  the  future  as  the  result  of  a  complete  understanding  of  this 
cosmical  pulsation.  The  attack  upon  the  problems  has  really  consisted  in 
rather  blindly  groping  for  the  most  sensitive  pulse  in  the  entire  cosmical 
circulation,  and  in  disentangling  the  several  interacting  t}-pes  of  impulses. 
It  is  e^-ident  that  the  partial  failures  hitherto  attending  this  work  have 
been  due  to  two  principal  causes:  (1)  The  comparison  was  made 
between  the  changes  in  the  spotted  areas  of  the  sun  and  the  terrestrial 
variations,  but  these  solar  changes  were  not  sensitive  enough  to  register 
a  complete  account  of  the  action  of  the  solar  output.  Discussions  of 
the  spots  are  being  replaced  by  others  upon  the  solar  prominences  and 
faculse,  which  respond  much  more  exactly  to  the  working  of  the  sun's 
internal  circulation:  (2)  The  magnetic  and  meteorological  observa- 
tions have  not  been  handled  with  sufficient  precision  to  do  justice  to  the 
terrestrial  side  of  the  comparison.  It  is  evident  that  all  these  physi- 
cal data  at  the  sun  and  at  the  earth  must  be  computed  with  an 
exactness  comparable  to  that  of  astronomical  observations  of  position, 
if  meteorology  is  to  be  raised  to  the  rank  of  a  cosmical  science.  When 
one  considers  the  crudeness  of  the  meteorological  data,  taken  the  world 
over,  due  to  the  character  of  the  instruments  employed,  the  different 
local  hours  of  observation,  and  the  divergent  methods  of  reduction, 
it  as  no  wonder  that  small  solar  variations  have  been  swallowed  up 
in  the  bad  workmanship  of  meteorologists.  The  prevailing  methods 
have  been  sufficient  for  forecasting  and  for  climatological  purposes,  but 
they  are  entirely  inadequate  for  the  cosmical  problems  whose  solution 
will  form  the  basis  of  scientific  long-range  forecasts  over  large  areas 
of  the  earth — that  is,  for  forecasting  the  seasonal  changes  of  the 
weather  from  year  to  year.  It  is  perfectly  evident  that  if  secular  varia- 
tions f)f  any  kind,  such  as  the  annual  changes  in  terrestrial  pressure, 
20 


292  THE    CLIMATE    OF    BALTIMORE 

temperature,  or  magnetic  field,  are  to  be  attributed  to  solar  action,  the 
original  observations  must  be  finally  reduced  to  a  homogeneous  system. 
The  local  peculiarities  of  each  station  must  be  carefully  eliminated, 
and  the  data  of  numerous  stations  must  be  concentrated  before  anything 
like  quantitative  cosmical  residuals  can  be  obtained.  When  we  consider 
that  .there  have  been  numerous  changes  in  the  elevations  of  barometers, 
various  methods  of  reducing  the  readings,  and  many  groups  of  selected 
hours  of  observations  entering  into  the  series  at  the  same  station,  how 
could  it  be  expected  that  anything  better  than  negative  results  in  solar 
problems  would  be  obtained?  The  skeptical  attitude  of  conservative 
students,  who  declare  that  the  many  indecisive  results  already  obtained 
mean  that  there  is  no  true  and  causal  solar-terrestrial  synchronism,  is, 
of  course,  quite  fallacious  until  it  has  been  demonstrated  by  the  use  of 
first-class  homogeneous  data  that  the  suspected  physical  connection  is 
imaginary.  There  is  but  little  question  that  the  existing  uncertainty  is 
in ^f act  based  upon  the  use  of  the  ver}^  imperfect  methods  of  observation 
and  reduction  which  have  prevailed  in  meteorological  offices,  rather  than 
upon  the  unreality  of  the  phenomena  in  nature." 

The  results  of  a  comparison  of  Baltimore  weather  observations  Avith 
the  sunspot  and  solar  prominence  frequency  curves  have  not  differed 
from  those  arrived  at  in  similar  investigations  elsewhere — they  neither 
prove  nor  disprove  an  intimate  relationship.  As  pointed  out  in  preced- 
ing paragraphs  there  are  synchronous  changes  here  and  there  in  the 
constantly  fluctuating  terrestrial  conditions,  but  on  the  whole  the  evi- 
dence is  negative.  In  view  of  the  complicated  character  of  the  weather 
conditions,  especially  in  our  middle  latitudes,  a  close  agreement  in  phase 
of  any  periodic  changes  need  scarcely  be  looked  for,  but  the  length  of 
the  period  of  the  terrestrial  and  solar  changes  should  harmonize. 

In  Fig.  84,  the  sunspot  and  solar  prominence  curves,  constructed  from 
Wolf's  tables  as  printed  in  the  Monthly  Weather  Eeview*  are  shown 
in  connection  with  curves  representing  the  actual  annual  changes  at 
Baltimore  in:   (a)  the  mean  pressure,   (&)   the  mean  temperature,   (c) 

♦Monthly  Weather  Review  of  the  U.  S.  Weather  Bureau  for  April,  1902. 


MARYLAND    WEATHER    SERVICE  293 

the  total  rainfall,  (d)  the  frequency  of  thunderstorms,  and  (e)  the 
fiequency  of  storm  winds  (exceeding  25  miles  per  hour).  In  Plate  XII, 
these  facts  have  been  presented  again  in  a  modified  form,  the  annual 
values  for  the  climatic  conditions  having  been  smoothed,  eliminating 
some  of  the  irregular  fluctuations  in  order  to  show  more  clearly  any 
periodic  occurrences  of  longer  period.  The  values  employed  in  the  con- 
struction of  the  curves  of  Plate  XII  were  computed  by  means  of  the 

following  formula,  ^^^^ TL^  in  which  a,  h,  and  c  represent  actual 

4 

values  for  three  successive  years.  In  this  manner  a  smoothed  value  was 
computed  for  each  year  of  the  entire  series. 

Plate  XII  contains  in  addition  a  record  of  all  excessive  rains  at  Balti- 
more from  1836  to  1904,  an  excessive  rain  being  defined  as  a  fall  of  2.50 
inches  or  more  in  24  consecutive  hours.  These  excessive  rainfalls  were 
taken  from  two  distinct  records;  those  occurring  from  1871  to  190-4  are  a 
part  of  the  official  record  of  the  U.  S.  Weather  Bureau;  those  of  the 
period  from  1836  to  1870  are  from  the  record  of  the  Army  Medical 
Department  at  Fort  McHenry,  with  very  few  exceptions.  The  rainfalls 
of  the  earlier  period  are  apparently  too  frequent,  owing  to  the  fact  that 
there  was  more  uncertainty  in  noting  beginnings  and  endings  of  precipi- 
tation than  in  the  later  series.  The  earlier  record  doubtless  contains 
excessive  rains  in  which  the  time  limit  was  extended  to  30  or  36  hours. 
However,  the  grouping  and  relative  frequency  of  these  excessive  falls  are 
the  features  to  which  especial  attention  is  called ;  the  actual  frequency 
is  of  less  importance. 

In  a  preceding  paragraph  reference  was  made  to  the  fact  that  the 
periods  of  excessive  frequency  of  heav}^  rains  coincided  very  closely  with 
the  periods  of  minimum  sunspot  frequency  from  1871  to  1901.  The 
earlier  observations  were  not  then  at  hand.  On  extending  tlie  series  of 
observations  back  to  1836,  the  same  nice  agreement  does  not  hold  good; 
there  is  a  gradual  change  to  a  reversal  in  the  phase  of  the  sunspot  period. 
However,  the  grouping  is  very  striking,  and  the  average  length  of  the 
periods  from  maximum  to  maximum,  or  minimum  to  mininnun.  agrees 
very  well  witli  the  average  length  of  the  sunspot  and  solar  prominence 
periods. 


(Smoothed  annual  values  for  weather  condifo"''    •  excessive  rainfalls,  which  are  observed  values.) 


MARTLAXD    WEATHER    SERVICE  395 

These  selected  Baltimore  observations  are  reproduced  in  Fig.  84  and 
Plate  XII,  not  so  much  to  call  attention  to  any  particular  cycle  of  changes 
as  to  place  tliein  into  convenient  form  for  a  critical  study  by  some  who 
may  later  find  a  more  suitable  clue  to  the  solution  of  the  difficult  problem 
of  the  relationship  between  solar  activity  and  terrestrial  weather  changes. 

General  Character  of  the  Seasons. 

The  average  values  of  climatic  conditions  and  the  departures  from 
these  values  have  been  discussed  in  considerable  detail  in  the  text  and 
tables  of  the  preceding  pages.  To  all  but  the  expert  in  the  study  of 
statistical  tables  it  is  a  difficult  matter  to  derive,  from  a  table  of  figures, 
no  matter  how  perfect  the  arrangement,  a  satisfactory  conception  of  the 
general  character  of  a  selected  period,  be  it  a  day,  a  month,  a  season,  or 
a  year.  The  graphic  method  of  presenting  results  appeals  to  a  greater 
number  because  it  enables  the  eye  to  take  in  at  a  glance  relations  between 
groups  of  values  which  would  be  more  or  less  obscure  to  the  casual  reader 
when  presented  in  tabular  form.  While  recognizing  the  limitations  of 
the  graphic  method  for  representing  such  a  complex  conception  as  the 
general  character  of  the  season,  it  has  yet  seemed  profitable  to  resort  to 
the  use  of  a  diagram  for  grouping  such  factors  as  are  deemed  most 
important  in  characterizing  the  weather  conditions  of  a  season. 

The  results  arc  siiown  in  Plates  XIII  to  XVII.  in  which  eight  selected 
factors,  expressed  as  departures  from  the  normal  climatic  conditions  at 
Baltimore,  are  presented  for  each  season  and  year  from  1871  to  1904. 
The  choice  of  factors,  as  well  as  their  arrangement,  was  a  purely  arbitrary 
matter,  hut  they,  as  a  iiiaitcr  of  course,  remain  the  same  for  each  season 
and  year.  Tlir  method  of  cliartiug  may  be  briefly  described  here, 
although  the  legends  on  the  plates  will  be  found  sufficiently  clear  for  this 
purpose.  The  normal  value  of  each  of  the  climatic  factors  selected  is 
represented  by  a  point  in  the  circumference  of  a  circle,  at  the  inter- 
section of  a  radius  representing  one  of  the  eight  points  of  the  compass. 
Taking  for  example  the  mean  seasonal  temperature:  A  departure  above 
the  normal  would  be  represented  by  placing  the  point  a  given  number 
of  units  beyond  the  circle  along  the  extension  of  the  radius  representing 


296  THE    CLIMATE   OF    BALTIMORE 

temperature ;  for  a  departure  below  the  normal,  the  point  would  be  placed 
within  the  circle  along  the  same  radius.  By  applying  a  similar  method 
for  each  of  the  factors  and  joining  the  points  thus  located,  we  have  a  char- 
acteristic octagonal  figure.  A  season  having  all  its  points  located  in  the 
circiimference  of  the  circle  would  be  represented  by  a  regular  octagon. 
The  degree  of  departure  from  the  regular  octagon  shows  at  a  glance  the 
amount  and  the  character  of  the  departure  from  normal  climatic  con- 
ditions of  the  season  inspected.  The  unit  adopted  for  measuring  the 
amount  of  departure  is  the  same  in  the  discussion  of  all  seasons  and 
years,  namely,  the  average  variability  of  the  factor.  The  average  varia- 
bility was  obtained  by  adding  up  the  individual  departures  for  each 
season  or  year  and  dividing  by  the  number  of  years  employed. 

The  normal  value  for  each  factor  is  shown  by  the  figures  given  below 
the  designation  of  the  factor  in  the  key  accompanying  each  set  of 
diagrams.  To  bring  the  form  representing  the  character  of  the  season 
into  further  relief  and  separate  it  from  the  scale,  the  former  is  tinted. 
The  comparison  of  the  seasons  at  Baltimore  from  1871  to  1904  will  be 
found  to  be  greatly  facilitated  by  the  use  of  these  diagrams. 

OBSERVATIONS  AND  INSTEUMENTAL  EQUIPMENT. 

Historical  Notes. 

Volume  I  of  the  Eeports  of  the  Maryland  Weather  Service  (1899) 
contains  a  report  upon  the  progress  of  meteorology  in  Maryland.  Eefer- 
ences  are  there  made  to  the  early  records  of  the  weather  which  came  to 
the  notice  of  the  writer,  and  to  the  development  of  systematic  instrumental 
observations.  While  temperature  records  were  regularly  kept  as  early  as 
1753  in  Prince  George's  County,  we  find  no  evidence  of  any  instrumental 
observations  within  the  limits  of  Baltimore  City,  or  in  the  immediate 
vicinity,  until  the  series  made  by  Capt.  Lewis  Brantz,  referred  to  in  the 
preceding  pages  in  connection  with  the  discussion  of  temperature  and 
rainfall.  To  the  best  of  the  writer's  knowledge  the  very  complete  and 
accurate  observations  of  Capt.  Brantz  were  the  earliest  made  within 
Baltimore  City. 


VOLUME  2,  PLATE  XIII. 


PRECIPITATION 
10    INCHES 


corner  of  the  plate.     A  departure  above   (  +  )   or  below   ( — )   the 
f  departure  (indicated  by  the  figures  1,  3,  5  along  the  radii)  is  the 


MARYLAND  WEATHER  SERVICE 


General  CHAnACTEn  of  the  Seasons.— Winter. 
average  seasonal  variability  of  the  factor.  °"'     ■  '■'=^P«'="^«'y.  =■"•  ^'o^g  a  radius  or  its  extension.    Tlie  unit  of  departure  (indicated  by  the  figures  1.  3,  6  along  the  radii)  is  the 


VOLUME  2,   PLATE   XIV. 


DAYS  WITH 
ECIPITATPON 


PRECIPITATION 
10  9  INCHES 


lor  of  tho  plate.     A  dpparture  above   (4-)   or  below   ( — )   the 
jparturc  (indirated  Ijy  the  figures  1,  3,  5  along  the  radii)  is  the 


General  Character  i 


Seasons. — Sprin 


Pointe  in  the  oircumferencp  of  the  circle,  at  the  Interaection  of  the  railii  (0),  represent  the  average  value  of  factors  enumerated  In  the  key  In  the  lower  right-hand  corner  of  the  plate.  A  departure  above  ( + )  or  below  (— )  the 
average  (or  normal)  value  of  the  factor  is  shown  by  the  position  of  points  .beyond  or  within  the  circle,  respectively,  and  along  a  radius  or  Its  extension.  The  unit  of  departure  (indicated  by  the  figures  1,  3,  5  along  the  radii)  is  Ihe 
average  seasonal  variability  of  the  factor. 


VOLUME  2,   PLATE  XV. 


PRECIPITATION 
'»«    INCHES 


coriipr  of  Iho  plate.     A  departure  above   (  +  )   or  below   ( — )   the 
of  departure  (indicated  by  the  figures  1,  3,  5  along  the  radii)  is  the 


General  Chakactee  of  the  Seasons. — Sujimer. 

Points  In  the  circumference  of  the  circle,  at  the  intersection  of  the  radii  (0).  represent  the  average  value  of  factors  enumerated  in  the  key  in  the  lower  right-hand  corner  of  the  plate.  A  departure  ahove  (  +  )  or  below  (— )  the 
average  (or  normal)  vahie  of  the  factor  Is  shown  by  the  position  of  points  beyond  or  within  the  circle,  respectively,  and  along  a  radius  or  its  extension.  The  unit  of  departure  (indicated  by  the  figures  1.  3,  5  along  the  radii)  is  the 
average  seasonal  variability  of  the  factor. 


VOLUME  2,   PLATE  XVI. 


DAYS   WITH 
ECIPITATION 


PRECIPITATION 
98     INCHES 


cornpr  of  tlio  platp.     A   doparturp  ahovp   (-f )   or  1)p1ow   ( — )    thp 
of  rlepartiirp  (  indicated  by  the  fiKiires  1.  3,  5  along  the  radii)  is  the 


Seasons. — Autumn. 


Points  in  thp  rircunitcrenrp  of  the  rircle.  at  the  inlorspotion  of  the  railii  (( 
veragp  (or  normal)  value  of  tlie  factor  is  .shown  by  the  position  of  points  tje 
verage  seasonal  variability  of  the  factor. 


below  I  —  )   the 


represent  thp  average  value  of  factors  enumerated  in  the  key  in  the  lower  right-hanil  corner  of  the  plate,     A  rleparture  above  (  +  ) 

ntl  or  within  the  circle,  respectively,  and  along  a  radius  or  its  extension.    The  unit  of  departure  (indicated  by  the  figures  1.  3.  6  along  the  radii)  is  the 


VOLUME  2,    PLATE   XVII. 


cornor  of  tlio  platp.     A  departure  above  (-|-)   or  below   (  — )   the 
if  departure  (indicated  by  the  figures  1.  3.  5  along  the  radii)  is  the 


PRECIPITATION 
«    INCHES 


Point 
average  ( 
average  seasonal 


I  the  circumferpnoe  of  the  circle,  at  the  intersection  of  the  radii  (0),  represent  the  average  value  of  factors  enumerated  in  the  key  in  the  lower  right-hand  corner  of  the  plate.     A  departure  above  (  +  )  or  below  ( — )   the 
normal)  value  of  the  factor  is  shown  by  the  position  of  points  beyond  or  within  the  circle,  respectively,  and  along  a  radius  or  its  extension.     The  unit  of  departure  (indicated  by  the  figures  1,  3,  5  along  the  radii)  is  the 
annual  variability  of  the  factor. 


MARYLAND    WEATHER    SERVICE  297 

Some  years  later,  in  1831,  a  station  of  the  U.  S.  Army  Medical  Depart- 
ment was  established  at  Fort  McHenry,  where  observations  were  main- 
tained with  but  little  interruption  until  1892.  Between  1830  and  1840, 
two  or  three  individual  Baltimore  observers  sent  occasional  reports  to 
the  Maryland  Academy  of  Sciences,  or  to  the  Franklin  Institute  in 
Philadelphia. 

The  Smithsonian  Institution  was  established  in  1847,  and  very  soon 
after  numerous  voluntary  observers  reported  weather  conditions  regularly 
from  different  parts  of  the  State.  The  Baltimoreans  who  cooperated  with 
the  institution  between  1850  and  1860  were  Dr.  A.  Zumbrock  (1850-52), 
Dr.  Lewis  H.  Steiner  (1853),  and  Prof.  Alfred  W.  Mayer  (1857-59). 

In  the  year  1870,  the  U.  S.  "Weather  Bureau  (then  known  as  the  IT.  S. 
Signal  Service)  was  established  by  act  of  Congress.  The  Baltimore 
station  was  among  the  first  to  be  opened,  and  observations  were  main- 
tained without  the  interruption  of  a  single  day  until  the  present  time. 
The  instrumental  equipment  was  always  of  the  first  order  and  was 
steadily  increased  with  the  growth  of  the  Bureau.  During  the  past  ten 
years,  continuous  records  of  air  pressure,  temperature,  wind  velocity  and 
direction,  sunshine,  and  rainfall,  by  means  of  self-recording  instruments, 
have  been  maintained.  In  1902,  a  Richard  hygrograph  was  added,  the 
property  of  the  Maryland  State  Weather  Service.  Details  concerning 
the  history  and  equipment  of  the  U.  S.  Weather  Bureau  station  are  given 
in  the  following  pages  in  tabular  and  statistical  form  for  ready  reference, 
including  changes  in  the  location  of  the  observing  station,  in  the  elevation 
of  instruments  and  in  tlie  personnel  of  the  station. 

In  1891,  the  Maryland  State  Weather  Service  was  organized;  the 
])urpose  and  method  of  organization  are  fully  described  by  the  director 
in  Volume  I,  of  the  Reports  of  the  Maryland  Weather  Service,  issued 
in  1899.  From  the  beginning  there  has  always  been  an  intimate  and 
harmonious  cooperation  between  the  National  Service,  the  State  Service, 
and  the  Johns  Hopkins  University.  The  offices  of  the  U.  S.  Weather 
Bureau  and  of  the  Maryland  Weather  Service  have  been  in  the  buildings 
of  the  University  since  the  establishment  of  the  State  Service.  The 
Board  of  Control  of  the  Maryland  Service  comprises  a  representative  of 


298 


CLIMATE  OF   BALTIMORE 


MARYLAND    WEATHER   SERVICE 


299 


METEOROLOGICAL  OBSERVERS  AND  OBSERVATIONS  IN  BALTIMORE,  MD. 


West  part  of  city  1         Private 


Fort  McHenry 


Observer 


Capt.  Lewis  Brantz 


Post  Surgeons,  L'.  S.  A. 


Dr.  G.  S.  Sproston,  U.  S.  N. 


Md.   Academy   of  Science 
and  Literature 


Dr.  T.  Edmondson,  .Jr.       1   West  part  of  city 

Dr.  A.  Zumbroclt 

Dr.  Lewis  H.  Steiner  Baltimore  Medical 

Institute 

Prof.  Alfred  M.  Mayer 

Wiu.  Lutber  Woods  !     .Johns  Hopkins 

Hospital 


Auspices 


North        Longitude    Elevation 
West  of 


Latitude 


Md.  Acad.  Sci. 
and  Lit. 


Johns  Hopkins 


*U.   8.    Weather   Bureau; 

{U.  S.  Signal  Service  un-j         University 
til  July  1,  ISiil) 


Smithsonian 
Institution 


Smithsonian 
Institution 


Smithsonian 
Institution 

Maryland  S.  W. 

S.  and  U.  S.  AV. 

Bureau 

United   States 
Government 


16  '  76 

17  76 

I 
17   I   76 


feet 


Period. 


50 
113 

+123 


1817-1834 

Jan.  to  Aug.,  1839 

Nov.,  1S36  to  June, 

1837 


1S31  to  June,  1859 

Apr.,  1«61  to  Jan.,  1863 

1864  to  Feb.,  1893 


B6,  21  and  33,  June, 
Sept.  and  Dec. 


1835  to  1837 
1846  to  Oct.   1853 


Apr.,  1850  to  July,  1853 
Jan.  to  Oct.,  1853 


Hours  of  Observation 


Items  Observed 


8  a.  m.,  3  p.   m.   and   \      Temperature,  rainfall,  winds,  '     Published  in  pamph- 
10  p.m.  clouds;  Barometer   and  hygro.  i  let    form   from   1818 

meter  added  in  1836.  i  to  1825 ;  reprinted  in 

I  the  American  Alma- 
nac, 1834,  and  in 
I  other  publications. 

7  a.  m.,  3  p.   m.   and         Temperature,  humidity,  wind 
9  p.  m.  direction  and  weather;   rainfall 

added  in  May,  1836. 


I.,  2  p.   m.  and 
9  p.  m. 


Hourly 


Sunrise,  10  a.  m., 
3  p.  m.  and  7  p.  m. 


■  a.  m.,  3  p.   m.  and 
y  p.  m. 


a.  m.,  3  p.  m.  and 
9  p.  m. 


Temperature, 
and  rainfall. 


Nov.,1857toAug.,  1859|  7  a.  m.,3p.   m.  and 
9  p.   m. 

^o^;  1894  to  dat 


■'«"■.  1871  to  date 


Self-registering  maxi- 
mum and  minimum 
thermometer 

See  page  303  for  houri 
of  observation 


'  For  earlier  locations  and  elevations  see  page  304. 


ometer  elevatioi" 


Occasional      reports 

to  Franklin  Institute 

of  Philadelphia. 

Pressure,   temperature,   wind  | 

direction   and  velocity,    hygro-  |     Published  in  Trans. 

meter,     cloud    movement    and    Md.  Acad.  Sci.,  Vol.  I, 

state  of  weather.  1837 

Pressure,  temperature,  wind  Printed  copies  of  re- 
direction and  force,  dewpoint,  ports  for  Jan.,  Mar., 
rainfall  and  weather.  July    and   Aug.    pre- 

sented   to    Maryland 
Academy  of  Science. 

Temperature,  wind  direction 
and  raiufall. 

Pressure,  temperature,  winds, 
clouds,  rainfall,  relative  humid- 
ity. 

Temperature,  wind  direction 
and  rainfall. 

Temperature,  wind  direction, 
weather  and  rainfall. 


Daily  observations  at  stated 
hours,  since  Jan.  1,  1&71  of 
pressure,  temperature,  wind 
direction  and  velocity,  humid- 
ity, clouds  and  rainfall;  tem- 
perature of  water  in  harbor, 
Sept.  1881  to  March,  1887;  at- 
mospheric electricity,  1882  to 
1887. 

Continuous  automatic  record 
of  wind  velocity  since  Jan., 
1871 ;  wind  direction  since  1874; 
rainfall  since  Nov.,  1893;  pres- 
sure and  temperature  since  Jan., 
1893  ;  sunshine  since  July,  1893; 
humidity  since  Feb.,  1903. 


300  THE    CLIMATE    OF    BALTIMORE 

each  of  the  three  institutions.  Since  1896,  the  system  of  voluntary 
observing  stations  established  in  1891,  and  later,  by  the  State  Service,  and 
the  publication  of  weekly  and  monthly  reports,  have  been  under  the  con- 
trol of  the  National  Service.  The  special  appropriation  by  the  State  has 
since  been  devoted  to  the  investigation  of  special  climatic  problems 
and  the  publication  of  the  results. 

The  local  office  of  the  U.  S.  Weather  Bureau  is  intimately  associated 
with  the  commercial  organizations  of  the  city.  The  results  of  observa- 
tions and  specially  prepared  reports  are  quickly  put  into  the  possession  of 
those  most  benefited  thereby  through  the  instrumentality  of  the  public 
press,  by  special  bulletins,  by  telephone,  or  otherwise.  Special  efforts 
have  always  been  made  to  place  the  daily  telegraphic  reports  of  weather 
conditions  from  all  parts  of  the  countrv'  before  the  commercial  interests 
at  the  earliest  possible  hour  each  day.  These  reports,  as  soon  as  received, 
are  placed  upon  a  large  glass  map  upon  the  floor  of  the  Chamber  of 
Commerce,  by  means  of  symbols  and  lines;  the  Baltimore  station  was 
among  the  earliest  to  adopt  this  method  of  publishing  the  daily  weather 
conditions. 

One  of  the  most  important  duties  of  the  local  office  of  the  Weather 
Bureau  is  the  prompt  distribution  of  the  daily  forecasts  to  the  public, 
and  of  special  warnings  of  the  approach  of  storms,  or  cold  waves,  or  the 
occurrence  of  frost.  This  is  accomplished  by  a  liberal  use  of  the 
telegraph  and  the  telephone,  and  by  the  hearty  cooperation  of  the  public 
press  and  the  U.  S.  Postal  authorities,  especially  through  the  instru- 
mentality of  the  recently  established  system  of  rural  free  delivery, 
by  means  of  which  the  daily  weather  forecasts  are  being  placed  regularly 
each  day  in  possession  of  all  who  dwell  along  the  established  routes,  even 
those  in  remote  farming  communities. 

The  shipping  interests  are  informed  directly  by  telephone  of  the 
approach  of  a  severe  storm;  the  owners  of  the  smaller  craft  in  the 
neighboring  waters  depend  mostly  for  their  information  upon  the  dis- 
play of  storm  warnings  at  a  conspicuous  point  along  the  harbor  by  means 
of  special  storm  flags  by  day  and  lights  by  night.  For  many  years,  these 
.  special  warnings  were  displayed  from  the  flag  staff  on  Federal  Hill ;  they 


MARYLAND  WEATHER   SERVICE. 


VOLUME  2,   PLATE  XVIII. 


OFFICE  OF  THE  U.  S.  WEATHER  BUREAU, 
JOHNS  HOPKINS  UNIVERSITY,   No.   532  NORTH   HOWARD  STREET. 

MARYLAND  STATE  WEATHER  SERVICE. 


MAETLAXD    WEATHER   SERVICE  301 

are  now  displayed  from  the  top  of  a  steel  tower,  50  feet  in  height,  erected 
upon  the  roof  of  the  seamen's  home,  known  as  "  The  Anchorage,"  at  the 
intersection  of  South  Broadway  and  Thames  Streets  in  East  Baltimore. 
Eeports  on  the  weather  and  crop  conditions  throughout  the  States  of 
Marjdand  and  Delaware  have  been  printed  and  widely  distributed  from 
the  local  oflSce  each  week  during  the  season  of  crop  growth  from  1891  to 
the  present  time.  Monthly  and  annual  reports  have  also  been  issued 
during  the  same  period,  showing  the  daily  weather  conditions  and  the 
monthly  and  annual  average  values,  together  with  the  progress  of  crop 
growth  and  farm  operations  from  month  to  month.  A  printed  daily 
weather  map  (single  sheets  11  in.  by  16  in.  in  size)  was  issued  from  the 
local  oflBce  from  October,  1896,  to  N'ovember,  1898.  These  received  a  free 
and  wide  distribution  in  the  business  portions  of  Baltimore  and  among 
educational  institutions.  The  maps  showed  the  weather  conditions  over 
the  entire  country,  based  on  over  a  hundred  telegraphic  reports  of  obser- 
vations made  each  morning  at  8  a.  m.,  75th  meridian  time.  They 
reached  the  public  about  1  p.  m.  each  day.  In  November  of  1898,  the 
printing  office  connected  with  the  Baltimore  station  was  closed  and  the 
daily  weather  map  was  replaced  by  the  larger  and  more  complete  litho- 
graphic map  issued  from  the  Central  Office  at  Washington,  D.  C. 

Observers  and  Observations. 
The  table  on  pages  298  and  299  contains  a  list  of  the  individuals  and 
institutions  responsible  for  systematic  instrumental  observations  of  the 
weather  within  the  city  limits  of  Baltimore.  The  geographical  location 
of  the  station,  the  period,  and  the  character  of  the  observations  are  also 
stated  as  far  as  known.  There  were  doubtless  other  observers  but  their 
contributions  to  the  observational  literature  of  meteorologv'  in  Baltimore 
have  not  come  under  the  notice  of  the  writer. 

Instrumental  Equipment. 
The  instruments  in  use  at  the  local  station  of  the  U.  S.  Weather  Bureau 
since  the  organization  of  the  Service  in  1871  are  enumerated  in  the 
following  list;  the  dates  of  installation  and  discontinuance  of  tlie  instru- 
ments are  also  jrivcn: 


302 


THE    CLIMATE   OF   BALTIMORE 


Instruments 


In  Use  Since 


Anemometer,  Robinson Jan.      1,  1871 

Anemoscope   Jan.      1,  1871 

Barograph,  Richard I  Dec.   30,  1892 

Barometer  (Mercurial) Jan.      1,  1871 

*Hygrograph,  Richard Feb.,         1902 

Hygrometer,  stationary Jan.      1,  1871 

Psychrometer  (whirling) July   11,  1886 

Rain  Gage  (ordinary)          Jan.      1,  1871 

Rain  Gage  (self-registeriug,  float) May    30,  1891 

Rain  Gage  (self- registering,  tipping  bucket)    ,  June  13,  1897 

*  Rain  recorder,  continuous Jan.,         1903 

Register,  single  (wind  velocity) i  Jan.      1,1871 

Register,  double  (wind  velocity  and  direction) Feb.  10,  1874 

Register,  triple  (wind  velocity,  direction  and  rain).  .  Nov.  10,  1892 

Snow  Gage Jan.      1,  1871 

Sunshine  Recorder  (added  to  triple  register) June  30,  1893 

Shelter,  standard  window ■  Jan.      1,  1871 

Shelter,  standard  roof I  Oct.      1,  1885 

Thermograph,  Richard j  Dec.   30,  1892 

Thermometer,  dry  bulb Jan.      1,  1871 

Thermometer,  maximum July  22,  1872 

Thermometer,  minimum July  22,  1872 

Thermometer,  water i  Sept.    1,  1881 

*  Property  of  the  Maryland  State  Weather  Service. 


Use  Discontinued 


July   11,  1886 


June  20,  1897 


Feb.    10, 
Nov.  10, 


1874 
1892 


Sept.  30,  1885 


Mar.  31,  1887 


Hours  of  Observation. 

The  prescribed  hours  of  observation,  as  well  as  the  kind  of  time  em- 
ployed in  the  National  Service,  have  been  changed  from  time  to  time 
since  the  organization  of  the  Bureau  in  1870.  Local  time  was  in  use  at 
all  stations  from  January  1,  1871,  to  July  31,  1881 ;  on  August  1,  1881, 
Washington  time  was  adopted,  making  a  difference,  of  two  minutes 
between  the  local  times  of  observation  in  Washington  and  Baltimore. 
Since  January  1,  1885,  observations  have  been  made  on  75th  meridian 
time.  The  difference  between  Baltimore  local  time  and  75th  meridian 
time  is  six  minutes,  the  former  being  slower  than  the  latter  by  this 
amount. 

The  combination  of  selected  hours  for  observation  has  varied  con- 
siderably in  the  thirty-four  years  since  1871.  From  1871  to  June  30, 
1888,  at  least  three  direct  observations  were  made  daily,  one  at  an  early 
morning  hour,  from  7  a.  m.  to  7  :30  a.  m.,  another  in  the  middle  of  the 
afternoon,  at  2  p.  m.,  3  p.  m.,  or  4 :30  p.  m.,  a  third  at  night  between  9 


MARYLAND    WEATHER    SERVICE 


303 


p.  111.  and  11 :3U  p.  m.  Five  observations  were  made  daily  for  some  years. 
Since  July,  1888,  there  have  been  but  two  observations,  one  at  8  a.  m., 
another  at  8  p.  m.  The  exact  hours  of  observation  and  the  duration  of 
their  employment  are  indicated  in  the  following  tabular  statement : 


HOURS  OF  OBSERVATION. 
(U.  S.  Weather  Bureau,  1871-1904.) 


Time  of  Observation. 


11:87  p.  u).  (1.  t, 
11:02  p.  m.  (1.  t 


7:00  a.  m.  (1.  t.) 
2:00  p.  m.  (1.  t.) 
9:00  p.  m.  (1.  t.) 


12:02  p.  m.  (1.  t.)  (special) Oct.    22, 

12:02  p.  m.  (I.  t.)    (daily) Feb.    23, 


.Telegraphic 


11:02  a.  m.  (1.  t.). 

7:02  p.  m.  (1.  t.) 

7:02  a.  m.  (1.  t.) 

7:00  a.  m.(w.  t.) 

3:02  p.  m.  (1.   t.) 

3:00  p.  m.  (w.  t.) 

11:02  p.  m.  (1.  t.) 

11:00  p.  m.(w.t.) 

2:00  p.m.   (w.  t.) 

2:00  p.  m.   (7.5th  m.  t.) 

7:(I0  a.  m.    (7.5th  m.  t.)^ 

3:00  p.  m.    (75th  m.  t.) 

11:00  p.  m.   (75th  m.  t.) 

10:00  p.  m.   (7.5tli  m.  t.)J 


Water  temperatures.  . 


Tehii^raphic 


11:00  a.  m.   (75th  m.  t.). 
7:00  p.  m.   (75th  m.  t.). 


8:00  a. 
K:00  p. 


(75th  m. 
(75th  m, 


!:;} 


Telefj^raphic 


Local  lime  (1.  t.)  6  min.  slower  than  75tli  m. .  . . 
Wasliin^^ton  (w.  t.)  8  iiiin.  slower  than  75th  ni. 
7.5th  meridian  time 


Ended. 


Oct.  31,   1879 

Oct.  31,   1879 

Aug.  24,  1872 

Dec.  31,   1884 


June  30,   1881 

Feb.,  1872 

Dec.    31,  1879 

Dec.    31,   1884 
Dec.    31,  1884 

July    31,   1881 
Dec.    31,   1884 

July    31,   1881 
Dec.    31,   1884 

July    31,  1884 
Dec.    31,  1884 

Dec.    31,  1884 
Mar.   31,    1887 

June  oO,   1888 
June  .30,   1888 

Dec.    31,   1880 
June  30,   1888 

Aug.     3,   1886 
Aug.     3,  1886 

Cnrrent 
Current 

July    31,    1881 

Dec.    31,   1884 

Current 


304 


THE    CLIMATE   OF    BALTIMORE 


Changes  iisr  the  Location  of  the  Station. 

Changes  in  the  location  of  the  observing  station  of  the  U.  S.  Weather 
Bureau  have  necessitated  changes  in  the  elevation  of  instruments.  During 
a  period  of  34  years^  five  different  stations  have  been  occupied.  The 
office  has  always  been  in  the  thickly  settled  portion  of  the  city;  from 
1871  to  1889  in  the  heart  of  the  business  section,  later  in  the  buildings 
of  the  Johns  Hopkins  University,  No.  532  North  Howard  Street.  The 
details  of  the  changes  experienced  in  station  and  instruments  are  indi- 
cated in  the  tabular  statement  below.  With  unimportant  exceptions, 
the  instruments  have  always  had  a  fairly  free  exposure,  as  good,  perhaps, 
as  can  be  had  in  the  midst  of  a  large  city. 


location  of  u.  s.  weather  bureau  stations  and  elevation 
of  instruments. 


ci 

Above  Ground  (Feet). 

o 
o 

Observations  begu 

3 

o 
12; 

a) 

a 
-c 

3 

'3) 

a 

a 

^•» 

—  V 

< 

Location. 

a 

2 

03 
P5 

• 

a 

o 

a 

n 
J3 

33 

76 

9 

6c 
08 

a 
"3 
P3 

69 

© 

a 

o 

a 
§ 

<! 

75 
86 

® 

a 

as 
■d 

c 

Fireman's  Insurance 
Building,  S.  W.  Cor. 
South*  Water  Sts 

3d 

Jan.     1, 1871 
Oct.    12, 1878 
Oct.     1, 1885 

39°  18' 

76°  37' 

45 

33 

85 
95 

Neal  Office  Building, 
S.  W.  Cor.  HoUiday  & 

4th 

Upper 

floor 

of 

tower 

Jan.     1, 1889 

June    1, 1891 
1893 

39°  18' 
390  18' 

76°  37' 
76°  37' 

76 
179 

56 
71 

86 

87 

78 

78 
80 

100 
100 

110 

Johns  Hopkins  Univer- 
sity. Physical  Labora- 
tory, N.  W.  Cor.  Linden 
Ave.  &  Monument  St. . . 

110 

Equitable  Building, 
S.  W.  Cor.  Calvert  & 
Fayette  Sts 

9th 

Sept.   7,1895 

39°  18' 

76°  37' 

143 

105 

120 

116 

136 

146 

Johns  Hopkins  Univer- 
sity, Treasurer's  Build- 
ing, No.  533  N.  Howard 
St 

2d 

Aug.    l,189fi 
Apr.  30,1902 
Oct.      1, 1903 

39°  18' 

76°  37' 

123 

20 

68 
69 

73 

83 
117 

94 
115 

♦Thermometers  in  louvered  window  shelter  on  north  side  of  building  from  Jan.,  1871  to 
Sept.  30, 1885  ;  later  in  standard  shelter  on  the  roof  of  the  station  building.  On  Oct.  1, 1902, 
the  standard  shelter  was  mounted  within  the  60  ft.  steel  tower  supporting  the  wind  vane 
and  the  anemometer,  the  base  of  the  shelter  being  nine  feet  above  the  roof. 


MARYLAND  WEATHER   SERVICE. 


VOLUME  2,   PLATE  XIX. 


z 

o 

o 

5 

1- 

1- 

< 

< 

1- 

CO 

1) 

> 

> 
< 

_l 

< 

Q 
< 

a. 

O 

05 

q: 

Q 

CD 

Li- 

o 

O 

z 

1- 

z 

o 
o 

CE 

< 

- 

^ 

uT 

o 

2 

< 

a. 
O 

O 

I 

h- 

o 

05 

z 

MAEYLAXD    WEATHER   SERVICE 


305 


VARIATIONS  IN  THE  ELEVATION  OF  THE  BAROMETER  AND  CORRECTIONS 
TO  THE  EPOCH,  JANUARY  1,  1900.* 

Barometer  above  reference  plarte 25.3  feet. 

Reference  plane,  top  of  iron  pipe  underneath  sidewalk  on  west  side  of  Howard 
street  opposite  Center  street,  transverse  station  No.  482  of  the  City  of  Balti- 
more Topographical  Survey,  above  mean  sea  level 98.0    " 

Station  elevation  of  barometer,  Jan.  1, 1900 123.3    " 


Date  of 
change. 


Building's  Occupied. 


1870,  Dec.  23 
1889,  Jan.     1 ! 

1891,  June   1  • 

I 

1895,  Sept.    7 

1896,  Aug.    1 

1899,  Jan.     1 


Southwest  corner  of  South  &  Water 

streets 

Southwest  corner  of  Baltimore  and 

Hollidaj-  streets 

Johns  Hopkins  University  (Physical 

Laboratory  I 178.8 

Southwest    corner   of   Calvert    and 

Fayette  streets U1.5 

Johns  Hopkins  University  (No.  532 

North  Howard  street) 


45.2 
75.9 


123.3 


^  3fe 


+30.7 
+102.9 
-37.3 

-18.2 


la 
111 

;g||    Sgc 


+78.1 
+47.4  -.054 


-55.5 
-18.2 


+  .063 
+  .020 


-.015 
-.015 
-.015 
-.015 
-.015 


-.103 

-.069 

+  .048 

+  .005 

-.015 
.000 


*  For  further  information  regarding  the  reduction  of  barometric  observations  see :  Bige- 
low,  F.  H.  The  Reduction  of  Barometric  Pressure  Observations  at  Station  of  the  United 
States  Weather  Bureau.  Vol.  II  of  the  Report  of  the  Chief  of  the  L'.  S.  Weather  Bureau  for  1900. 


OFFICIALS  IN  CHARGE,  U.  S.    WEATHER  BUREAU  OFFICE,  BALTIMORE. 

(1871-1904) 


Name. 


Official  title. 


Cowan,  J.  E.  .  . 
Penrod,  II.  J.  . 
Boyd,  W.  T.  .  . 
Kabernagle,  J. 

Bell,   R.  J 

McCiann,   E.   W. 

Black,  W 

Seyboth,  Robt. 
Felfrer,  G.  W.  . 
Cronk,  C.  P.  .  . 
Marburv,  J.  B. 
Hunt,  G.  E.  ... 
Walz,  F.  J.  .  . . 
Fassig,  O.  L.  . . 


Sergeant 


Local  Forecast 
Official 

Section  Director 


Date  of 
assignment. 


Jan. 

May 

Sept. 

April 

Dec. 

May 

June 

Oct. 

Sept. 

Oct. 

July 

July 

May 

July 


1871 

1871* 

1874 

1875 

187.5 

1877* 

1879* 

1879* 

1882 

1888 

189.5 

1896 

1897 

1900* 


Date  of  relief. 


Mar.  27,  1871 
Sept.  23,  1874 
April  24,  1875 
Dec.  20,  1875 
May  21,  1877 
June  6,  1879 
Aug.  29,  1879 
Sept.  6,  1882 
Oct.  26,  1888 
July  6 
July  1 
May  22 
May  29,  1900 
In  charge 


1895 
1896 
1897 


•  During  intervening  intervals  the  station  was  in  charge  of  the  first  assistant. 


306 


THE    CLIMATE    OF    IJALTIMOKE 


3  T  "  £■  S 


iz 

iC 

ii 

N 

CO 

8 

C5 

s 

O 

£2 

CO 

1 

^ 

t- 

s 

30 

^ 

o 

1- 

le 

lO 

^ 

t- 

-* 

O 

^ 

OO 

s 

5 

(M 

s 

^ 

-* 

TO 

•^ 

f2 

lO 

s 

CO 

<o 

o 

ei 

O   O   O    O    fl 


MAltYLAXD    WEATHER    SERVICE 


30^ 


«o   ^   ;s       s: 


—    -rf      X 


O    tS     3D        00 
TJ   l-    3C       M 

3C    ^    O        >0 


X   —   r; 

«  5 


'^ 

?! 

N 

IJ 

— . 

lA 

■^ 

.=< 

" 

*~ 

o 

Q 

«c 

O    00 

55 

o 

o 

o  o 

ao  X 

o 

30 

=  =  ^ 

CO  t^  d  -H 

i;; 

»o 

ss 

CO  1-1 

>0    O    C'J       • 

-  CO  t-  o 

> 

o 

s 

O    C5 

30 

5^J 

o 

o  o 

o 

CO 

d 

«c 

c:  m  m 
c:    3C   cs 

N  d  d  o 

CO 

^ 

g^ 

00    t- 

s:  2  2^5    • 

.    O    C5    O 

o 

O 

» 

g 

f-l  o 

N    O 

is 

o 
d 

I- 

« 

w  d  d  Oi 

CO 

»l 

S^-Ss 

^ 

:  E^  H  ® 

ft 
o 
(» 

o 

E 

S 

t- 

00  o 

o  o 

OS 

N 

■* 

o 

»0    ^^    C5 

30    lO    O 

CO  d  d  C5 

s 

« 

sgg 

s 

O    CO    -*    CO 

<N 

be 
3 
< 

o 

s 

« 

•<* 

»-  o 

CO   CO 
00    lO 

o 

00 

d 

'O'  a>  o  ^ 

»! 

CO 

O    CO   -* 

O    CO    CO 

f-  ^  ^  ^ 

N 

July 

o 

CO 

« 

c? 

^ 

2  ^ 

CO    -* 

o 

S 

I- 

^^^ 

X 

t- 

8S  S 

~ 

S  '^  "^  "^ 

>o 

c 

■  s 

° 

c 

lO 

3:   o 

i-  fi 

t^ 

■* 

" 

ci  30'  d  o 

CO 

co 

S5S 

s 

lO    ^    >*     N 

'^ 

o 

5s 

CO 

« 

d 

t-  o 

F-l    30 

©1 

OS 

o 

N 

55  S  o 

CO    t^    rH    »3 

r. 

lO 

3  t^  »i  CO 

:  EH  H  => 

ft 
< 

05 

o 

°5 

E 

c^ 

d 

S.I  o 

-*   o 
>0    .-1 

OJ 

d 

d 

CO 

t-  o  >- 

(N   I-    CO 

CO    30    r^    -H 

CO 

•* 

8S 

00   o 

•  d  00  o 

lO 

o  ^ 

3i 

lO 

o 

o  o 

o  =g 

CO 
CO 

d 

05    ■*    OS 
05    »    -; 

CO    t^   -^   M 

C3 

00 

8S  S 

(M   d    CO    ^ 

2 

00    CO 

•  lO  d  o 

"zi 

cc 

O    5= 

!C 

o 

o 

o  o 

«o  o 

2 

oc 

30 

O    t-    lO 
l-    O    CO 

CO  t^  d  -^ 

S 

■* 

°^ 

lO   05 

-. 

s 

O   uj 

30 

o 

o 

o  o 

30    t- 

5SS 

«s 

CO 

d 

30 

CO  d  d  ©J 

2 

>* 

^ 

_     CO     CO    r^ 

;  lo  to  d 

5  + 


O  Si  +  c 

^  >  o  C 

^  2  §  ^    „■ 

5  £  °  *2 


2      i-   c 


•  2  ^ 

£  ^  o 

I  d  a,  g 

w  5  -2  § 

K  03  OS  i- 

Oi  0/  ei  J* 


&    £ 


OJ    o   S   sj   o 


'Z  1 


S   a   CJ 


.  =s 

C3     . 

— 

r-    ■ 

- 

c    . 

53 

C3     • 

^ 

^     • 

>i 

>.    • 

a   ■ 

«— > 

1— t 

^ 

s   . 

S^ 

o 

.   X) 

^  ■ 

a. 

O     . 

•  r^ 

'    V- 

V-       ' 

C 

O     . 

'X 

tc 

M   • 

c  • 

•  a 

■  A 

J3    • 

•  U 

^  : 

•  <i-i 

-  o 

o    • 

•   >, 

.    o 

o    . 

—    .       tn 


c    „  < 


=.<    <J 


a  s 


:^5_2   5g^;^~   -'- 


S  "3. 


tS    —    .—    1.    ■^,  "   —  (-1 


O       O  T    o 


o   a   5-   1.   a 


->■.>» 


r   i   c  ..    5, 


-■^^—   -^'      -f-o 


-  ^   -      - 

-  1)  S     c 
S  if  =    s 


.t;  H 


2  q 


-   c  —   - 


^  a  ~   o  o 

~  CS      fl  Q 

t;  ""  a  -  .£' 

§  °  M  £  S 

=  -2  >.  a  B 

C  =3  -Z    5  ® 

I  i  I  I  S 

B  .=  -  I  E 

X  X  a   X  c 

::  a  1)   «  i 


308 


THE    CLIMATE   OF   BALTIMORE 


<M  -^ 


le  ic  sc  -* 

CO    O    CC    C5 


CJ    SI    "O 
tr^    00    lO 


lO   CO  tc  ■*  o 
-^  S  CO  •* 


©5   f-t    CO    00    *-< 
00    <-l    O    C3    00 


i-H    N   05    O    O 


»-  t-  I—  I-l 


OO    M  ■* 

•*  to  ro 


>0    CB   CD    ■*    rt 


1-1  "-I         N 


O   >0    03    CO   >0 


o 

is 

05  O  -*  O  -^ 

00 

CO 

?2 

§ 

1 

to   o 
»o  o 

03 

05 

o 

CD 

e» 

OS 

CD 

CO 

OS  CO  ■* 

■t-3 
O 

O 

CO 

-t. 

■  00  l- 

00 

o 

-* 

ei 

t- 

:  :  :  :  EH 

CO 

s 

2 

^ 

CO  00 

■* 

US 

«■» 

g? 

t- 

o 

CO 

■* 

00  «o  o 

-p 

■* 

(M 

i-H  C! 

>a 

CO 

OS 

lO 

to 

<g 

5 

s 

§ 

3 

OO  05 

CO 

-* 

s 

S3 

co 

o 

s 

■* 

t-  lo  o 

C3S   00  o   :c 


1-    CO    t-    t-    O'l 


I-H     ■*     O 

OS  »i  CO 


c»  o 

-*  O  OSN    C5 


-1   c:  t-  -*  C5 


CO    CO    ^    U3 


O-t    -*    i-H 

cs  »i  CO 


U5    OS    t-    05  •>* 


r-l  1— I  C-l 


lO    CO    t-    CO    f-1 


:  H 


CO  oi  o  cc 


t-    I-   CO 

t-^   OS   ■* 


00    lO  CO 

lO    OS    >0    CO    'H 


OS  lO  OS   ^-  -^ 


^' 

-* 

o 

^ 

s 

CR 

o 

IC 

tr- 

« 

■X. 

rt 

p. 

N 

o 

Ol 

o 

X 

§ 

g 

■-"" 

- 

to 

io 

OS 

^ 

-*  .-1 

^ 

CO 

OS 

00  <N 

•^ 

I-} 

o 

OS 

00 

» 

l- 

CO 

00 

lO 

t- 

S 

N 

CD 

o 

I^! 

CO 

s 

CO 

00 

»o 

>o 

00 

-* 

■*  « 

00 

lO 

o 

CO  lO 

-* 

u: 

lO 

^1 

■* 

-* 

CO 

o 

-* 

OS 

OS 

CO 

CO 

s 

o 

lO 

s 

s 

CO 

00 

-* 

»o 

00 

>o 

CO  o 

s 

■<* 

00 

CD  e) 

d 

(S 

>-5 

CO 

OS 

o 

o 

L^ 

58 

OS 

-* 

CO 

co 

OS 

CO 
00 

to 

00 

U3 

CO 

o 

CO  CO 

o    E-( 


I     & 


S    ..^ 


u  o  " 

<a  a  fl 

— "  —"  o 

o  o  a 

th  iH  ■" 


""-I  "r'  ~ 

&    &    K 

O    O    "M 


Q  Q  g.  a 

J;  Oi  O  t»-( 

■§-§  ^  ^ 

a  s  0)  o 

3  3  4=  a 

15  ^  a  M 

i-  ci  £0  c3 

o  o  ej  0) 

>  t-  o  • 


-5  -S 


a 


cJ  rt 

>-;  "3  "3  S 

t^  ^  u 

"?  5  •=>  S 

'S  9  c:  c3 

S  g  5£ 
§S 


So 


O     O   02 

03  m  (^ 


o  o  K 

0)0  0 

a  s  j; 

z  z  s 


m  Z 


ci  '5;  ^   es 


>  -^  o  ■~'       ^    2Lzi  o  SL 


>-i  C3  -l-i  -1-1 
w  *  <u  « 
3    <1  O  h3 


"?  es  oj  to  -=; 

JQQ  ^O 

'3  u  tJ  D  ^ 

S  cs  ci  S 

O  a)  o  ti  fc< 

r1  ~)  Ej  '^  i* 

Cj  ^  '-^  o  Ch 

<"  •"  "y  o  <i-i 

a  S  "  °  o 

=  -2  a  =-  -2 

c  a  a  o  a 

3  a  5  "2  a 

<;  Z  Z  a  2; 

C3  cj  -S  4->  n 

t.  1^  cj  m  t^ 

Q)  o  a>  ci  s- 


2    to 

-o  Q 


a  a  >> 
■y  *  <w  --  r 


o  Pl, 


s  < 


>    t-i    a;    r* 


a  o  a  a  o 

a  /3  3  -  — 

!^  a  Z  Z  a 

«  Z  f*  «  Z 

Mo 

cj  M  t-  eS  K 

o)  c3  a)  o  ci 

U  QJ  >  U  <2^ 

O  iJ  <!  C  w; 


MARYLAND    WEATHER    SERVICE 


309 


»o   c->  c-1  o  t-      ci 


»©    O    t-    lO        t-    !C    Ir- 


CI   -^  iO 


«■ 

o 

22.0 
1.9 
.00 
3.00 

NW 

^ 

1** 

: 

d  -1  o  1-1 

> 
o 

^H  (M  "-H 

«o  5-  S  8  > 

^ 

1^* 

:  : 

d  N  O  rt 

.w 

^ 

-<* 

«  O  05  '^  t- 
1-^  W 

^ 

1** 

00  t- 

s 

■*  !0 

2 

25 

35 

d  I-}  o  o 

4i 
c 
© 

CO 

t-  t-  S  n 

«0  O   '  rt  ^ 

^ 

s»s= 

rt  lO  O  1-1 

C'^  C-l  ..*  IC  x< 

»1  ^  O  ^  3C 

-*  »  S  S  ;> 

2°  ■'^^ 

cs 

»  s* 

: 

•  '■ 

o 

^  o  o  o 

3 

^  I-  CC  O  I- 

rr  t;  o  I-  X 

2°     °S^ 

^ 

1^* 

=  ~ 

>o 

;^  = 

s 

-# 

M- 

^    is^    1-1   C:^ 

1-5 

OS 

^^1^^  : 

■  • 

to 

lO  .-1  r-l  CO 

1 

03  C3  00  »  00 

21  «  5  "*  "^ 

>0  M  O  N  ^ 

^ 

l.. 

00 

CO  00  O  r-c 

(4 

< 

lO 

§  1  2  g  § 

^ 

^ 

1^1 

o  ao 

o 

S  *J 

'«' 

.* 

>o 

IN 

i-i  «0  O  CO 

t-  lO  O-l  lO  t- 

•O  M  §  S  ^ 

-*  >o  d  re  ^ 

^  ^^  : 

•  : 

d  -*  o  CO 

2 

o  o 
o  ca  o  -*  ^ 

N          1^ 

Jz; 

i^^  ■■ 

-* 

d  ci  o  CO 

CO 
>0  O  S5  CO  TO 

(N  05  S  S  fe 

g3  ^  -=  "  z 

^^^    : 

IN  t- 

CO 

-^  N 

2 

CO 

a> 

d  1-1  o  -* 

a  o  o  t,  2 
S  s  a  S  ■ 

3    o    cj    *-*  O 


"5  "^  ^' 

O  O    1^    3 
r-l  n  O    „    3 


3    S   ^ 
rt    ^    ^ 


y    u  o  o  r 
J    J  P  p  5 


^    w    3 


i  a  5  ^ 
'^333 

?i   =    3    3 


_o  _o  _o  g  •-  — 

"qj  "o  "c  c  o  *    o 

>  >  >  ^  ^  £ 

O  O  O  g  -I  "-I 


5  a  Q  o 

t«  to  bi:  !i< 


3    3       3    3    3    a> 


^  Z  0  (»  02 


M  |S  Z 


i-'  r'  ^  •-'  ^  w 
Z  **  5  «  5  ^ 
"  •sj  ^  i«  .<  fc< 


3  3 

^  S  2, 

a   «  a) 

m    f.*  t." 

3  fa  b 
C  a! 

£  a  a  s 

3  2^ 

am  7J 

oa    3  c    e 

u    3  3  -S 

-3  t^  r  >. 

H  5  ja   §. 

2:'  >■  §  §1 

P    3    „  = 

S5  I  s  = 

H  S  = 

o  3  -  .3  -r 

Z   03  X  e   ~ 

g   O)    ts  i!    c 


MARYLAND  WEATHER   SERVICE. 


VOLUME  2,   PLATE   XXL 


to 


-r  O 

I-  --  UJ 

<     .  E 

Oil  ^ 

oo  ^ 

§?  = 

uj2  > 

Oz  I 

I-  —  °- 


THE  WEATHER  OF  BALTIMORE 


INTEODUCTION 


Leaving  now  the  discussion  of  climate^,  or  the  average  and  extreme 
values  of  the  principal  factors  which  constitute  the  sum  total  of  atmos- 
pheric conditions,  we  come  to  a  consideration  of  some  of  the  more  im- 
portant types  of  weather  characteristic  of  the  geographical  horizon  of 
Baltimore.  As  stated  in  the  introduction  to  this  report,  the  term  weather 
is  restricted  in  its  use  to  the  actual  state  of  the  atmosphere  as  regards 
temperature,  humidity,  wind  movement,  etc.,  at  any  given  instant,  or 
short  period  of  time.  The  method  employed  in  the  discussion  of  cli- 
matic conditions  is  not  applicable  to  descriptions  of  weather;  the  various 
factors  cannot  be  considered  separately,  but  must  be  studied  in  their 
relations  to  one  another  at  a  given  instant  of  time,  in  order  to  afford  a 
proper  mental  picture  of  actual  conditions  in  nature. 

The  past  fifty  or  sixty  years  have  witnessed  a  gradual  but  radical 
change  in  our  views  of  weather  conditions  and  sequences.  Before  the 
days  of  the  telegraph,  observers  were  isolated  and  independent  of  one 
another;  we  had  but  vague  ideas  as  to  synchronous  weather  conditions 
prevailing  at  distant  points.  Here  and  there  in  the  eighteenth  century 
we  find  a  suggestion  of  the  importance  of  co-operation  in  the  methods 
and  time  of  making  observations ;  but  intercommunication  was  slow  and 
the  important  discoveries  of  Franklin  and  Jefferson  in  America,  of  Bran- 
des  in  Germany,  and  others,  met  with  rather  tardy  recognition,  or  were 
entirely  overlooked  and  had  to  be  rediscovered  when  times  were  more 
propitious  for  utilizing  the  results  of  new  discoveries. 

The  rich  collection  of  weather  proverbs,  based  upon  natural  signs — 
changes  in  the  wind,  forms  of  clouds,  the  habits  of  animals — are  based 
upon  the  accumulated  experience  of  individual  effort.  For  centuries 
21 


312  THE    CLIMATE    OF   BALTIMORE 

weather  changes  were  minutely  observed  and  carefully  recorded.  Espec- 
ially was  this  true  of  changes  in  wind  direction,  as  this  factor  was  almost 
universally  regarded  as  the  underlying  cause  of  variations  in  the  other 
elements.  Not  until  the  use  of  the  telegraph  became  general,  making  it 
possible  to  gather  reports  from  an  area  covering  thousands  of  square 
miles,  and  to  obtain  a  picture  of  actual  weather  conditions  at  the  same 
instant  of  time  over  this  area,  was  the  true  meaning  of  weather  changes 
gradually  revealed  to  the  student  of  meteorology.  Change  in  the  direc- 
tion of  the  wind,  while  it  still  holds  a  conspicuous  place  in  weather 
prognostics,  is  no  longer  regarded  as  the  fundamental  factor  in  the 
weather  situation.  The  weather  map,  so  familiar  to  us  to-day,  shows  us 
that  atmospheric  pressure,  or  the  height  of  the  barometer,  is  the  key  to 
the  problem  of  coming  weather — not  the  actual  height  of  the  barometer 
at  a  single  station,  or  at  a  number  of  stations,  but  the  relative  heights 
over  a  large  area.  Having  given  the  relative  distribution  of  pressure 
over  a  given  area,  the  remaining  weather  elements  can  be  supplied  with 
a  fair  degree  of  accuracy  by  the  expert  student  in  weather  forecasting. 

The  Synoptic  Weather  Chart. 
The  development  of  the  synoptic  weather  chart,  a  chart  showing  the 
actual  physical  condition  of  the  atmosphere  at  the  same  hour  over  an 
area  of  thousands  of  square  miles,  forms  one  of  the  most  interesting 
chapters  in  the  history  of  modern  meteorology.  Conceived  before  the 
close  of  the  first  quarter  of  the  nineteenth  century,  the  middle  of  the 
century  witnessed  a  remarkably  rapid  development  and  application  of 
the  idea.  This  was  brought  about  by  the  rapid  spread  of  the  electric 
telegraph  and  the  recognition  of  the  vast  commercial  importance  of  such 
a  chart.  The  successive  steps  of  its  progressive  development  in  this 
country  and  in  Europe  have  been  carefully  traced  by  Abbe  ^  and  Hell- 
mann,"  to  whom  we  are  indebted  for  much  of  our  accurate  history  of 
meteorology. 

^  Abbe,  Cleveland.  The  Development  of  the  Daily  Weather  Map.  Vol.  I, 
Maryland  Weather  Service,  Baltimore,  1899,  pp.  225  et  seq. 

^  Hellmann,  G.  Neudrucke  von  Schriften  und  Karten  iiber  Meteorologie 
und  Erdmagnetismus.     No.  8,  Berlin,  1897. 


MARYLAND   WEATHER    SERVICE  313 

To-day  the  great  majority  of  the  nations  of  the  world  support  a 
national  weather  service  and  issue  such  charts  daily.  We  now  have 
daily  charts  of  synchronous  observations  for  most  of  the  land  area  of 
the  northern  hemisphere  excepting  Asiatic  Eussia  and  China;  and  of 
the  North  Atlantic.  In  the  southern  hemisphere,  there  are  charts  for 
Australia,  the  Indian  Ocean,  South  Africa,  and  the  Argentine  Eepublic. 
The  time  will  soon  come  for  the  realization  of  one  of  the  fondest  hopes 
of  the  meteorologist,  when  we  shall  be  able  to  construct  such  maps  for 
practically  the  entire  globe.  The  importance  of  the  constant  endeavor 
to  extend  the  area  of  observations  becomes  apparent  when  we  realize 
that  there  are  no  definite  boundaries  in  the  atmosphere  of  the  globe,  and 
that  no  extensive  disturbance  can  take  place  in  any  portion  of  its  vast 
extent  without  affecting,  sooner  or  later,  every  other  portion.  We  proba- 
bly do  not  yet  realize  fully  the  nice  adjiistment  of  atmospheric  forces, 
and  we  are  still  ignorant  of  many  of  the  important  laws  underlying  the 
larger  atmospheric  movements. 

Cyclones  and  Anti-cyclones. 
Before  the  advent  of  the  synoptic  weather  chart,  the  constant  changes 
in  the  direction  of  the  Avind,  the  increase  and  decrease  in  cloudiness,  the 
occurrence  of  rain  with  a  certain  wind  and  clear  skies  with  another, 
were  very  little  understood.  Certain  sequences  in  weather  changes  had 
long  been  accurately  noted,  but  why  these  changes  should  follow  a  defi- 
nite order  was  incomprehensible.  The  weather  map,  however,  revealed 
the  clue  to  the  interpretation  of  the  changes  in  the  relative  distribution 
of  atmospheric  pressure  over  extended  areas.  It  was  soon  learned  iliat 
differences  of  pressure,  or  of  the  height  of  the  barometer  in  neighboring 
localities,  set  the  air  in  motion,  causing  it  to  flow  from  the  area  of  high 
barometric  pressure  to  the  areas  of  lower  barometric  pressure,  much  as 
water  is  transferred  from  a  higher  to  a  lower  level.  This  flow  of  the  air 
from  place  to  place  in  an  effort  to  restore  a  disturbed  equilibrium  is 
what  is  termed  the  wind.  Changes  in  wind  direction  in  turn  bring  about 
changes  in  temperature,  the  wind  blowing  warmer  with  a  change  from  a 
northerly  to  a  southerly  direction  in  the  northern  hemisphere,  with  inter- 


314  THE    CLIMATE    OF   BALTIMORE 

mediate  changes  in  temperature  wlien  blowing  from  tlie  east  or  west.  A 
study  of  the  weather  map  soon  led  to  the  formulation  of  a  new  set  of 
rules  of  weather  changes,  more  general  in  their  application  and  more  in- 
telligible than  those  based  on  the  study  of  observations  at  a  single  sta- 
tion. Two  distinct  types  of  weather  were  soon  recognized.  The  most 
conspicuous  of  these,  and  the  first  to  be  investigated  was  the  storm  area, 
an  area  in  which  the  readings  of  the  barometer  decrease  rapidly  from  all 
sides  to  a  minimum  in  the  center  of  the  area.  Such  areas  were  observed 
to  be  accompanied  by  cloudy  skies,  and  more  or  less  rain,  by  winds  in- 
creasing in  force  as  the  center  was  approached,  and  blowing  approxi- 
mately toward  the  point  where  the  barometric  pressure  was  lowest.  As 
the  types  of  weather  first  investigated  were  naturally  well  developed 
storm  areas,  the  winds  high  and  blowing  in  paths  nearly  circular,  or  at 
least  spirally  inward,  the  term  cyclone  was  applied  to  them;  a  term  first 
used  about  the  middle  of  the  nineteenth  century  by  Captain  Henry 
Piddington  to  describe  revolving  storms  in  the  Indian  seas.  Later,  the 
term  cyclone  was  given  to  all  atmospheric  disturbances  of  wide  area  in 
which  the  winds  blow  toward  a  central  point  or  line  of  low  pressure. 

Another  weather  type  which  later  claimed  the  attention  of  students 
of  meteorology  was  the  fine  weather  type,  in  which  the  barometric  pres- 
sure is  highest  in  the  central  area,  decreasing  in  all  directions  from  the 
center  outward.  In  these  areas  the  winds  were  observed  to  blow  in 
general  away  from  the  center  of  highest  pressure.  As  the  character- 
istics of  these  areas  were  in  many  respects  the  opposite  of  those  observed 
in  cyclones,  they  were  given  the  name  anti- cyclones. 

Between  these  two  well  defined  types,  there  are  innumerable  forms  par- 
taking more  or  less  of  the  characteristics  of  one  or  flie  other  of  the  two 
principal  types. 

In  the  northern  and  southern  hemispheres,  from  latitude  30°  to  70°, 
the  upper  portions  of  the  atmosphere  apparently  flow  in  a  constant 
stream  in  a  direction  approximately  from  west  to  east  around  the  globe. 
The  winds  within  the  lower  layers  of  the  atmosphere  in  the  middle  lati- 
tudes do  not  always  follow  the  course  of  the  upper  currents;  the  atmos- 
phere from  the  earth's  surface  to  the  level  of  the  highest  clouds  is  broken 


MARYLAND  WEATHER  SERVICE  315 

up  into  areas  in  which  the  barometer  is  alternately  low  and  high,  into 
cyclones  and  anti-cyclones,  as  they  are  generally  designated.  These 
alternate  areas  of  unsettled  weather  and  tine  weather  are  carried  along 
as  a  whole  in  an  easterly  direction  with  the  general  drift  of  the  upper 
atmosphere,  constantly  changing  in  form  and  intensity  as  they  move,  but 
retaining,  in  the  main,  their  chief  characteristics  for  thousands  of  miles ; 
the  areas  of  high  pressure  are  accompanied  by  comparatively  little 
cloudiness,  and  temperatures  below  the  seasonal  average;  while  the  areas 
of  low  pressure  are  attended  by  clouds  and  rain  and  a  temperature  above 
the  seasonal  average.  These  cyclonic  and  anti-cyclonic  areas  of  the  middle 
latitudes  have  a  diameter  varying  from  a  few  .hundred  to  a  thousand,  or 
even  two  thousand,  miles  and  move  eastward,  as  a  whole,  with  a  velocity 
averaging  about  600  miles  per  day,  across  tlie  United  States,  and  hence 
occupy  two  or  three  days  in  passing  a  fixed  point.  Their  passage  east- 
ward explains  the  constant  shifting  in  the  direction  of  the  winds  of  a 
given  locality,  the  direction  of  the  change  in  wind,  as  will  be  explained 
later,  depending  upon  the  position  of  the  center  of  the  anti-cyclones  and 
cyclones  with  reference  to  the  given  locality.  For  instance,  when  a 
"  low,"  or  cyclonic  area,  approaches  Baltimore  from  the  west,  if  the 
center  is  north  of  the  latitude  of  Baltimore,  the  wind  becomes  easterly ;  as 
the  center  passes  Baltimore,  the  wind  shifts  to  the  south,  and  then  to  the 
west  or  northwest.  If  the  center  of  the  storm  passes  to  the  south  of  the 
city,  the  changes  in  the  wind  direction  are  successively,  east,  north,  and 
west,  the  reverse  of  those  in  the  first  case  cited.  If  the  center  passes 
over  Baltimore,  the  easterly  wind  is  followed  by  a  calm,  or  light  variable 
wind,  after  which  the  wind  will  spring  up  abruptly  from  the  west. 

These  rules  of  weather  sequences  are  applicable  only  to  the  well  devel- 
oped and  definitely  formed  areas  of  high  and  low  pressure,  and  but  im- 
perfectly apply  to  the  more  numerous  moderately  developed  "  highs  "  and 
"  lows "  whose  eastward  drift  gives  us  our  daily  routine  of  weather 
changes. 

A  clear  conception  of  the  character  of  cyclones  and  anti-cj'clones,  as 
described  above — of  the  distribution  of  pressure,  the  system  of  winds, 
the  distribution  of  temperature,  tlie  state  of  the  weather,  and  the  move- 


316  THE    CLIMATE    OF    BALTi:\IORE 

ments  of  these  areas,  as  a  whole — is  essential  to  a  proper  understanding 
of  our  daily  weather  changes,  and  especially  of  the  more  conspicuous 
weather  types  known  as  storms  and  cold  waves.  The  essential  features 
of  cyclones  and  anti-cyclones  may  be  most  readily  understood  by  a  study 
of  actual  examples  of  the  simpler  well  developed  types.  As  an  illustra- 
tion of  a  typical  storm  or  cyclone,  the  weather  chart  of  the  morning  of 
December  27,  1904,  is  reproduced  in  Figs.  85,  86.  To  those  unfamiliar 
with  the  weather  majD — and  indeed  to  all  excepting  those  who  have  de- 
voted much  time  to  their  study — the  usual  weather  map  is  a  confused 
tangle  of  lines  and  symbols  requiring  careful  explanation  and  analysis 
before  even  the  essential  features  of  the  weather  conditions  are  under- 
stood. Some  of  these  difficulties  may  be  obviated  by  the  use  of  a  series 
of  charts  portraying  the  separate  factors  which  go  to  make  up  the  com- 
plex weather  conditions,  retaining  in  each  chart,  however,  the  controlling 
factor,  namely,  the  system  of  lines  representing  the  distribution  of  atmos- 
pheric pressure,  or  isobars,  as  they  are  called. 

AREAS   OP   UNSETTLED  WEATHER    (CYCLONES). 

It  may  be  well  at  this  point  to  call  attention  to  the  very  frequent 
misuse  of  the  terms  cyclone  and  tornado.  In  scientific  literature  there 
is  a  clear  distinction  between  the  two,  while  in  the  popular  mind,  they 
are  often  synonymous.  The  confusion  in  the  use  of  these  terms  is  natural, 
and  is  largely  due  to  a  change  in  the  meaning  of  the  word  "  cyclone  " 
as  used  in  technical  literature.  A  cyclone  has  at  all  times,  since  the  days 
of  Piddington  (about  1850),  been  regarded  as  a  severe  and  destructive 
storm.  Later  when  the  natiire  of  storms  and  weather  changes  was 
better  understood,  the  meaning  of  the  term  was  enlarged  by  the  student 
of  meteorology  to  include  all  atmospheric  disturbances,  whether  large 
or  small,  intense  or  barely  perceptible,  in  which  the  winds  blow  inward 
toward  a  central  point,  or  area  of  low  barometric  pressure.  Tlie  general 
public  has  not  yei  adopted  this  amended  definition,  and  all  intense 
storms  continue  to  be  called  cyclones,  when  the  particular  storm  in  mind 
may  be  a  tornado,  squall,  an  intense  thunderstorm,  or  a  hurricane.  While 
the  tornado,  as  a  revolving  storm,  must  be  classed  with  cyclones,  the 


■;r  service 


baouboiqsi  gjisrio  lariJBSw  x^iBb  .8  /J  oJ  aailqqe  nobfinBlqxa  sniwoIIoH  9riT 
Jb  9f)jBm  snoitBvieado  giroeaBlIumra  no  baaBd  91b  eJiBrio  sriT     JioqsT  siriJ  ni 

.9mii  flBibiiam  riJ6Y  ,.m  .s  8 
.(.irfB'?  8991395  ni)    9'iiiiBieqtnel  Isupe  lo  89nil  sib  asnil  jlosia 
.(aerfoai  ni )    eiuaaaiq  oiierfqaomJB  lBup9  5o  39nil  91b  asnil  b9fl 
Fir.  ii5.— T-??i^ai^?«^»5?*)yft'  iff  g:M£ttiiaftiiJ  3lBm,'(8B9iB  b9bBfl8 

.bniw  offt  rfJiw  xft  awoiiB  9dT 

.niBi  asiBoibni  H 

.wona  asJBOibni  8 

21   3nib909iq   s^J  gniiuh    rmoJaiebnnrii  6   lo   9oa9ni;90o   9rfJ   89:tB0lbfli  T 


by  the  iv 

LEGEND. 

The  following  explanation  applies  to  U.  S.  daily  weather  charts  reproduced 
in  this  report.  The  charts  are  based  on  simultaneous  observations  made  at 
8  a.  m.,  75th  meridian  time. 

Black  lines  are  lines  of  equal  temperature    (in  degrees  Fahr.). 

Red  lines  are  lines  of  equal  atmospheric  pressure    (in  inches). 

Shaded  areas  mark  the  limits  of  overcast  skies. 

The  arrows  fly  with  the  wind. 

R  indicates  rain. 

S  indicates  snow. 

T  indicates    the    occurrence    of    a  thunderstorm    during   the    preceding    12 

hours. 

}  clone  ^' 
-,..,.:  the  '^  — 

■1     rlcc;ff^,. 


'  ^enerfil 


MARYLAND    WEATHER    SERVICE 


317 


Fig.  85.— Typical  Cyclone  of  December  27,  1904. 


Fio.  86.— Typical  Cyclone  of  December  27,  1904. 


318  THE    CLIMATE    OF   BxVLTIMORE 

term  is  restricted  to  the  intense  and  very  destructive  local  storms  of  very 
small  area  occurring  within  certain  limited  portions  of  a  larger  storm; 
it  is  a  cyclone  within  a  cyclone. 

The  accompanying  charts,  Figs.  85  and  86,  show  the  distribution  of 
pressure,  wind  direction,  temperature,  cloudiness,  and  precipitation, 
within  the  area  of  the  typical  cyclone  which  passed  over  the  United 
States  during  December  27,  1904. 

Pressure  and  Winds. — The  series  of  red  curved  lines  show  the  dis- 
tribution of  atmospheric  pressure.  The  inner,  nearly  circular,  line  en- 
closing the  word  "  low  "  in  the  chart  for  December  27  is  drawn  through 
localities  in  which  the  barometer  read  29.40  inches,  the  lowest  reported 
pressure.  The  remaining  curves  connect  localities  in  which  the  barome- 
ter stood  higher  by  successive  intervals  of  two-tenths  of  an  inch,  until 
around  the  outer  limits,  at  approximately  a  thousand  miles  from  the 
center,  the  barometer  read  30.40  inches,  showing  a  difference  in  pressure 
of  one  inch. 

The  atmosjihere  is  very  responsive  to  local  differences  of  pressure.  With 
very  slight  differences,  a  small  fraction  of  an  inch,  for  example,  there 
will  be  set  up  a  movement  of  air  from  the  locality  having  the  higher 
toward  that  having  the  lower  reading  of  the  barometer  until  equilibrium 
is  restored,  for  the  same  reason  that  water  always  has  a  tendency  to 
flow  from  a  higher  to  a  lower  level.  Bearing  in  mind  this  general  physical 
law  of  the  flow  of  gases,  we  may  understand  the  general  drift  of  the 
atmosphere  toward  the  central  area  of  low  pressure  in  a  cyclone.  This 
is  clearly  shown  by  the  arrows  which  indicate  the  direction  of  the  wind 
at  so  many  stations  of  observation;  the  arrows  fly  with  the  wind  and 
point  in  a  general  way  toward  the  area  of  lowest  pressure.  As  will  be 
observed  they  only  occasionally  point  directly  toward  the  center;  in 
most  instances  the  direction  taken  by  a  particle  of  the  atmosphere  in  its 
journey  from  the  outer  portion  of  a  storm  area  toward  the  center  is 
along  a  spiral  course.  This  is  due  to  the  effect  of  the  rotation  of  the 
earth  about  its  axis,  which  always  tends  to  urge  a  freely  mo'ving  particle 
toward  the  right  of  its  initial  direction,  in  the  northern  hemisphere. 
The  topography  of  the  region  over  which  the  storm  passes,  and  more 


]MARYLAND   WEATHER   SERVICE  319 

important  still,  the  distribution  of  the  pressure  about  the  central  area  as 
shown  by  the  curved  lines,  or  isobars,  also  greatly  influence  the  direction 
of  the  wind.  In  general,  and  especially  over  land  areas,  the  isobars  vary 
widely  from  the  circular  form,  and  the  actual  wind  directions  fall  roughly 
into  two  classes,  easterly  and  westerly.  Drawing  a  north  and  south  line 
through  the  center  of  the  cyclone,  the  winds  to  the  east  are  observed  to 
flow  mostly  from  some  point  between  northeast  and  southeast,  while 
those  to  the  west  of  the  line  blow  mostly  from  some  point  between  south- 
west and  northwest.  The  winds  blowdng  directly  from  the  south  or  from 
the  north  are  not  so  frequent,  or  of  as  long  duration  as  those  from  other 
directions.  The  effect  of  pressure  distribution  on  the  direction  and  force 
of  the  \\-'w.d  will  be  brought  out  very  clearly  in  later  discussions  of 
weather  types.  In  general,  it  may  be  stated  that  the  force  of  the  wind 
is  greater  the  greater  the  difference  of  pressure  between  two  neighboring 
points ;  that  is,  it  is  proportional  to  what  is  called  "  the  gradient,'^  which 
is  equivalent  to  difference  of  level  in  the  flow  of  streams;  the  steeper  the 
bed  of  the  stream,  the  more  rapid  is  the  flow  of  water. 

Temperature  and  Wind  Direction. — Especial  attention  is  directed  to 
the  relation  existing  between  wind  direction  and  temperature  in  a 
cyclone.  Localities  reporting  the  same  temperature  at  8  a.  m.  are-  joined 
by  means  of  lines,  or  isotherms,  as  they  are  styled.  The  lines  are  drawn 
at  intervals  of  10°  or  20°  Fahr.  It  will  be  observed  that  the  temperature 
of  70°  above  zero  in  lower  Florida  gradually  diminishes,  as  we  proceed 
northward,  to  30°  below  zero  in  the  extreme  northern  limits.  This  is 
the  usual  direction  of  decrease  in  temperature  at  all  seasons  of  the  year, 
though  tlie  rate  of  decrease  is  here  much  more  rapid  than  under  normal 
conditions.  In  the  absence  of  a  well  defined  atmospheric  disturbance 
the  isotherms  run  nearly  parallel  with  the  lines  of  latitude;  a  steady  and 
fairly  uniform  decrease  in  temperature  from  south  to  north  is  the  normal 
condition  all  over  the  northern  hemisphere.  The  relative  temperature 
of  winds  from  different  quarters  may  vary  greatly  in  different  localities, 
but  in  the  main,  a  southerly  wind  is  warmest  and  a  northerly  wind 
coldest,  with  intermediate  degrees  for  the  east  and  the  west  wind.  Hence 
we  may  readily  understand  how  a  change  in  the  direction  of  the  wind 


320  THE    CLIMATE    OF   BALTIMORE 

may  affect  the  temperature  of  a  locality.  On  the  approach  of  a  cyclone, 
in  most  cases  from  the  west,  the  winds  to  the  east  of  the  center  become 
easterly,  veering  to  southerly;  the  temperature  rises  in  the  eastern  half 
of  the  storm,  and  particularly  in  the  southeast  quadrant  where  the 
winds  are  from  southeast  or  south.  In  the  southwest  quadrant  of  the 
storm  there  is  usualh-  an  abrupt  change  from  the  warm  south  or  south- 
east wind  to  a  much  colder  west  or  northwest  wind.  This  condition  of 
temperature  distribution  is  strikingly  exhibited  in  the  charts.  The 
isotherms  are  seen  to  bend  northward  far  beyond  their  seasonal  values 
in  the  southeast  quadrant;  on  the  other  hand,  in  the  southwest  quadrant, 
the  cold  northwest  winds  are  carried  far  beyond  their  normal  limits  to 
the  southward.  The  result  is  a  stronger  contrast  in  temperature  from 
the  center  of  the  storm  westward  than  from  south  to  north ;  the  isotherms 
run  north  and  south,  in  the  particular  case  cited,  and  not  east  and  west 
as  in  normal  weather  conditions.  These  shifts  in  the  direction  of  the 
wind  during  the  passage  of  a  cyclone  are  sufficient  cause  for  the  great 
majority  of  the  temperature  changes  experienced  in  our  latitudes. 

Distribution  of  Clouds  and  Precipitation. — The  proportion  of  the  area 
covered  by  clouds  at  8  a.  m.  is  indicated  by  the  extent  and  intensity  of 
the  shading.  It  will  be  observed  that  in  practically  all  of  the  region 
within  the  influence  of  the  system  of  closed  isobars,  the  skies  were  en- 
tirely overcast,  and  that  over  an  area  extending  500  to  600  miles  in 
nearly  all  directions  from  the  central  point  of  lowest  pressure,  precipi- 
tation occurred  at  the  hour  of  observation — rain  to  the  south  of  the 
isotherm  of  30°  and  snow  north  of  this  line.  The  area  of  precipitation 
in  this  particular  storm  was  exceptional,  but  it  serves  to  illustrate  the 
general  law  of  the  distribution  in  well  developed  storms  of  large  extent. 
In  most  cases  the  area  of  precipitation  is  more  limited  in  extent  and  is 
surrounded  by  a  band  of  overcast  skies,  beyond  which  is  a  partly  clouded 
band  merging  into  regions  of  clear  skies.  Not  all  cyclones,  even  when 
well  developed,  show  the  symmetry  in  the  distribution  of  weather  con- 
ditions indicated  in  the  type  selected;  the  variations  are  infinite,  but 
there  is  a  general  conformity  to  the  type  when  the  storm  is  well  developed. 
The  center  of  the  rain  or  snow  area  is  usually  to  the  east  and  south  of 


MARYLAND      WEATHER   SERVICE  321 

the  center  of  low  pressure.  The  details  of  the  distribution  and  the 
character  of  the  preciiDitation  will  be  brought  out  more  clearly  in  the 
later  discussions  of  weather  types  of  the  season. 

The  elements  described  separately  in  the  preceding  pages  are  finally 
brought  together  upon  a  single  chart,  the  usual  form  of  presenting  in  our 
daily  weather  charts  the  actual  condition  of  the  weather  at  a  stated  hour. 
Such  charts,  showing  the  actual  state  of  the  weather  at  8  a.  ra.  throughout 
the  United  States  and  the  British  Provinces  to  the  north,  are  issued  daily 
about  11  a.  m.  by  the  United  States  Weather  Bureau,  based  on  telegraphic 
reports  from  about  175  stations. 

AREAS   OF   FAIR   WEATHER    (aNTI-CYCLONES)  . 

As  already  stated  in  a  preceding  paragraph,  the  term  anti-cyclone  was 
first  used  to  describe  a  weather  type  which  shows  characteristics  just 
the  opposite  of  those  of  the  cyclones.  The  pressure  is  highest  in  the 
centre  of  the  area,  the  winds  blow  in  a  general  direction  away  from  the 
center,  the  skies  are  mostly  clear  to  partly  clouded,  with  little  or  no 
precipitation,  the  temperatures  are,  in  general,  loiver  at  the  center  than 
on  the  eastern  or  western  sides  of  the  area ;  that  is,  their  isotherms 
curve  southward  toward  the  center  of  high  pressure  while  those  in  the 
cyclone  bend  northward  toward  the  central  area  of  low  pressure. 

A  typical  example  of  an  anti-cyclonic  system  is  shown  in  the  weather' 
map  of  April  4,  1904,  reproduced  in  Figs.  87,  88.  It  occupied  approxi- 
mately the  same  position  and  covered  the  same  area  as  did  the  cyclone 
of  December  27,  1904,  shown  in  Figs.  85,  86. 

Isobars  and  Winds. — The  inner  circle,  or  isobar  of  30. GO  inches,  marks 
the  central  area  of  the  anti-cyclonic  system,  from  which  there  is  a 
steady  and  uniform  decrease  in  the  height  of  the  barometer  outward  in 
all  directions,  the  successive  isobars  marking  intervals  of  two-tenths  of 
an  inch  in  the  height  of  the  l)aromoter,  as  in  the  case  of  the  cyclonic 
system  described  above.  It  will  be  observed  that  the  gradient,  or 
steepness  of  the  successive  steps  between  isobars,  is  less  in  the  anti- 
cyclone than  in  the  cyclone :  the  area  covered  by  each  system  is  approxi- 
mately the  same,  wliilo  tlio  total  difference  in  pressure  between  tlie  center 


322 


THE    CLIMATE    OF    BALTIMORE 


Fig.  87. — Typical  Anti-cyclone  of  April  4,  1904. 


LOW 


Fig.  88. — Typical  Anti-cyclone  of  April  4,  1904. 


LEGEND. 

The  following  explanation  applies  to  U.  S.  daily  weather  charts  reproduced 
in  this  report.  The  charts  are  based  on  simultaneous  observations  made  at 
8  a.  m.,  75th  meridian  time. 

Black  lines  are  lines  of  equal  temperature    (in  degrees  Fahr.). 

Red  lines  are  lines  of  equal  atmospheric  pressure    (in  inches). 

Shaded  areas  mark  the  limits  of  overcast  skies. 

The  arrows  fly  with  the  wind. 

R  Indicates  rain. 

S  indicates  snow. 

T  indicates  the  occurrence  of  a  thunderstorm  during  the  preceding  12 
hours. 


IMORE 


.aviaoaj 

b^^ubo^qe^  siiBdo  ladisevr  ^liBb  .8  .U  oJ  asilqqE  noiiGaBlqxs  gniv/ollol  9riT 
is  9bBm  8noiJBvi98do  auosnBlIumia  no  baaBd  sib  sJibiId  sriT     Jioqai  sidJ  ni 

.sffliJ  flBibiisra  riJ6T  ,.ai  .b  8 

.(.idB'K  899is9b  fli)   9iuJjsi9qm9J  lBup9  lo  ssnil  91b  asnil  >lDBia 

.(asrioni  ni)   eiuaesiq  oiisriqaomJB  lBup9  5o  89ai{  91b  asnil  b9fl 

i>'i- — i  yp.ae!j(a' 16B3t9'7^  5o  atifhiT '^rf J  jiTfim  aB9'iB  b9bBri8 

.bniw  9ri}  rfJiw  yR  bwoiib  9rIT 

.ni£i  a9:tB9ibni  H 

.wona  89iBDibai  8 

SI   snib909iq>  sdi  sniiub    cmo:t2i9l)niJri}   b   Jo  eoaa-nvooo   9riJ   89iA0ibat  T 

.STUOfl 


Fig.  85.— Typi.  clone  of  April  4,  1904. 


MARYLAND   WEATHER    SERVICE  333 

and  outer  circumference  in  the  case  of  the  anti-cyclone  is  only  half 
that  in  the  cyclone,  namely,  one-half  an  inch.  This  is  also  shown  by 
the  number  of  the  isobars,  the  cyclone  having  double  the  number  shown 
in  the  anti-cyclone,  while  the  successive  steps  of  increase  or  decrease  in 
pressure,  and  the  entire  areas  covered  by  the  two  systems  are  the  same. 
This  is  characteristic  of  the  two  types,  though  the  proportions  may 
vary  greatly.  The  winds  are  observed  to  blow  in  a  general  direction 
away  from  the  center.  As  the  winds  in  an  anti-cyclone  are  in  general 
much  lighter  in  force  than  in  a  cyclone,  the  actual  directions  recorded 
near  the  surface  are  influenced  to  a  greater  extent  by  local  topography. 
This  is  especially  marked  near  the  center  of  the  area  where  the  winds 
are  very  light. 

The  law  of  deviation  of  a  particle  of  air  to  the  right  of  the  initial 
direction  flowing  from  the  center  of  the  high  area  outward  holds  good, 
as  noted  in  the  discussion  of  the  inward  flow  in  cyclones;  hence  the 
arrows,  indicating  the  direction  of  the  wind  in  the  anti-cyclone,  are 
observed  to  point,  not  directly  from  the  center  but  to  the  right  of  the 
radial  path,  the  angular  deviation  depending  upon  the  configuration  of 
the  isobars,  the  topography  and  other  factors. 

The  Winds  and  Distribution  of  Temperature.  Especial  attention  is 
directed  to  the  relation  between  wind  direction  and  temperature.  Here 
again,  as  in  the  case  of  the  cyclone,  the  temperature  is  seen  to  be  directly 
dependent  upon  wind  direction.  In  those  portions  of  the  area  where 
the  winds  are  mostly  from  a  warm  southerly  direction,  particularly 
noticeable  in  the  northwest  quadrant  in  the  illustration  shown,  the 
isotherms  are  bent  far  northward  of  their  normal  position  for  the  season. 
In  the  northeast  quadrant  of  the  area,  where  the  winds  are  mostly  from 
the  colder  north,  the  isotherm  of  30°  is  seen  to  dip  far  to  the  south. 
Along  the  Atlantic  coast  the  cold  of  the  northerly  wind  is  considerably 
tempered  by  the  presence  of  the  ocean,  over  which  the  rate  of  change  in 
temperature  is  much  less  marked  than  on  land  along  a  north  and 
south  line. 

In  the  center  of  a  "  high,"  another  factor  enters  to  lower  the  tem])era- 
ture  below  the  normal  for  the  season.     Here  the  skies  are  clear  and 


324  THE    CLIMATE    OF   BALTIMORE 

radiation  from  the  surface  of  the  earth  is  rapid.  This  is  especially  true 
during  the  night  hours  and^  in  consequence,  the  effect  is  quite  marked 
on  a  chart  based  on  observations  made  at  8  a.  m.,  before  the  heating 
effect  of  the  rising  sun  becomes  marked.  As  a  result  of  the  conditions 
described,  the  lowest  temperatures  in  a  well  developed  anti-cyclone  are 
generally  observed  to  be  within  the  central  isobar  of  a  system,  if  meas- 
ured by  departures  from  the  normal  seasonal  values  for  a  given  locality. 

Distribution  of  Clouds. — The  chart  selected  as  an  example  of  an  anti- 
cyclone shows  almost  too  well  one  of  the  characteristic  features  of  this 
type  of  weather,  iiamely,  the  absence  of  clouds.  While  freedom  from 
precipitation  and  clouds  is  the  most  striking  difference  between  a  cyclone 
and  an  anti-cyclone,  it  is  not  usual  to  see  a  weather  chart  with  a  high 
area  of  so  great  an  extent  with  skies  practically  free  from  clouds,  as 
is  here  shown.  Over  an  area  embracing  all  the  states  and  the  Canadian 
Provinces,  from  the  Mississippi  Valley  eastward  to  the  Atlantic  coast, 
an  overcast  sky  was  reported  at  8  a.  m.  from  only  four  or  five  observing 
stations.  Usually,  even  in  the  well  defined  and  well  developed  areas  of 
high  pressure  there  is  a  fair  percentage  of  cloudiness,  excepting  within 
a  radius  of  two  hundrea  ^r  three  hundred  miles  from  the  center. 

The  presumption  is  that  this  freedom  from  clouds  and  precipitation 
in  an  anti-cyclone  is  due  to  a  descending  atmosphere,  with  attendant 
increase  of  temperature,  due  to  compression  and  consequent  decrease 
in  the  relative  humidity;  the  opposite  process,  namel}^,  a  rising  atmos- 
phere cooled  by  expansion,  accompanied  by  an  increasing  relative  humid- 
ity and  by  cloud  formation,  and  later  by  rain  or  snow,  marks  the  cyclone. 

THE    EASTWARD    DRIFT    OF    CYCLONES    AND    ANTI-CYCLONES. 

In  describing  the  distribution  of  presfeure,  winds,  temperature,  and 
clouds  in  typical  c3^clones  and  anti-cyclones  in  the  preceding  paragraphs 
no  reference  was  made  to  their  movement  as  a  whole.  The  systems  are 
not  stationary  for  any  length  of  time.  In  addition  to  the  internal 
circulation  of  the  winds  described,  the  entire  systems  are  carried  east- 
ward by  the  general  drift  of  the  upper  atmosphere  in  the  middle  lati- 
tudes, retaining  at  the  same  time  their  chief  characteristics  as  cyclones 


MARYLAND    WEATHER    SERVICE 


325 


and  anti-eycloues  for  many  hundreds,  and  sometimes  thousands  of  miles. 
The  small  whirls  formed  in  a  rapidly  flowing  river  and  carried  down 
stream  with  the  general  current,  while  at  the  same  time  maintaining  their 
own  gyratory  motion,  are  often  cited  as  illustrations  of  the  drift  of 
cyclones  and  anti-cyclones  in  the  general  eastward  flow  of  the  atmos- 
phere between  the  parallels  of  30°  and  70°  north  and  south  latitude. 
These  "  highs  "  and  "  lows  "  do  not  extend  to  a  great  altitude,  but  are 


Fig.  89. — Typical  Cyclone  and   Anti-cyclone  of  March   3,  1904. 

formed  apparently  only  in  the  lower  portions  of  the  atmosphere,  the 
great  majority  of  them  being  confined  to  the  atmospheric  strata  below 
the  highest  mountain  ranges.  They  are  carried  in  an  easterly  direction 
in  the  middle  latitudes  with  a  varying  velocity  but  averaging  about  600 
miles  per  day.  wliile  moving  across  the  United  States.  There  is  a  con- 
stant and  rapid  succession  of  these  atmospheric  whirls,  or  waves  of  high 
and  low  pressure,  in  the  winter  and  spring  season.  In  summer  and 
early  fall  they  arc  loss  frequent  and  not  so  well  developed.     There  is 


326  THE    CLIMATE    OF   BALTIMORE 

an  excellent  example  of  a  well  developed  "  low  "  or  cyclone,  followed  by 
a  "high."  or  anti-cyclone  in  the  chart  for  March  3,  1904.     (Fig.  89.) 

The  shifts  in  the  direction  of  the  wind  experienced  during  the  passage 
of  these  "  lows "  and  "highs "  may  be  illustrated  upon  this  chart  by 
noting  the  directions  of  the  wind  at  points  along  a  given  parallel  of 
latitude  passing  from  east  to  west.  The  nature  of  the  change  of  wind 
depends  upon  the  position  of  the  center  of  the  cyclone  or  anti-cyclone 
with  reference  to  the  parallel  selected.  Taking  the  latitude  of  40 '^  for 
example  in  the  chart  for  March  3,  1904,  we  have,  as  we  pass  westward 
from  the  Atlantic  seaboard,  first  a  southeast  wind  followed  by  a  small 
area  of  south  wind,  in  AVest  Virginia,  followed  by  a  northwest  wind  to 
the  center  of  the  succeeding  "  high  "  in  western  Nebraska.  To  the  west  of 
the  center  of  the  anti-cyclone  we  have  again  a  southerly  wind  in  Colorado 
and  Utah.  Selecting  a  parallel  of  latitude  north  of  the  center  of  the 
"  low  "  and  "  high,"  we  have  a  reversed  order  in  the  shift  of  the  wind. 
In  the  "  low "  the  change  is  from  easterly  to  westerly  by  way  of  the 
north ;  and  in  the  "  high  "  from  westerly  to  easterly  by  way  of  the  south. 

Similar  shifts  in  the  wind  are  experienced  in  a  fixed  locality,  such  as 
Baltimore,  for  example,  as  these  cyclones  and  anti-cyclones  approach 
and  pass  beyond  the  observing  station.  An  easterly  wind  at  Baltimore 
heralds  the  approach  from  the  west  or  southwest  of  a  more  or  less 
developed  cyclone,  or  storm  area,  followed  by  increasing  cloudiness  and 
rain  or  snow,  as  the  center  of  the  storm  approaches.  After  the  wind 
veers  to  the  south  and  then  to  the  southwest  the  precipitation  soon 
ceases,  the  solid  cloud  mass  begins  to  break  into  patches  of  cloud  and, 
as  the  wind  gets  into  the  west,  the  proportion  of  clear  sky  increases  until 
the  cyclonic  system  passes  beyond  the  horizon  in  the  east.  This  is  the 
usual  order  of  change.  With  the  path  of  the  storm  center  to  the  south 
of  Baltimore,  the  wind  backs  from  east  to  west  by  way  of  the  north. 

The  weather  types  described  above  are  of  unusual  symmetry.  The  forms 
met  with  in  our  daily  routine  of  weather  conditions  are  infinite  in  variety. 
No  two  are  exactly  alike  in  all  their  details  of  pressure,  temperature  and 
cloud  distribution,  or  in  the  paths  pursued,  but  there  are  many  easily 
recognizable  types  with  marked  family  resemblances  which  are  of  great 


MARYLAND   WEATHER   SERVICE 


327 


assistance  to  the  practical  meteorologist  engaged  in  weather  forecasting. 
Some  of  these  types  will  be  discussed  in  detail  in  the  following  pages. 

WEATHER   CHARTS   OF   THE   NORTHERN   HEMISPHERE. 

A  remarkable  series  of  daily  weather  charts  covering  a  period  of  ten 
years  was  prepared  and  the  results  published  under  the  auspices  of  the 


Fig.  90. — Pressure  Distribution  over  the  Northern  Hemisphere,  Dec.  4,  1886. 


United  States  Weather  Bureau,  then  known  as  the  Signal  Service,  from 
1878  to  1887.  The  charts  were  based  on  reports  received  from  co-operat- 
ing national  weather  services  and  covered  the  whole  of  the  northern 
hemisphere  between  the  latitudes  of  about  20°  to  65°,  excepting  the 
22 


328  THE    CLIMATE    OF   BALTIMORE 

Pacific  Ocean.  One  of  these  cliarts,  sliovving  tlie  actual  distribution  of 
pressure  at  noon,  Greenwich  time,  for  December  4,  1886,  is  reproduced 
in  Fig.  90.  The  chart  shows  clearly  the  manner  in  which  the  lower 
atmosphere  of  the  middle  latitudes  is  segregated  into  successive  areas 
of  low  and  high  pressure,  or  cyclones  and  anti-cyclones  at  a  given  hour. 

Weather  of  the  Prixcipal  Climatic  Zones. 

It  has  been  customary  for  convenience  to  divide  the  surface  of  the 
globe  into  three  climatic  zones,  the  tropical,  the  temperate,  and  the  polar, 
separated  by  fixed  parallels  of  latitude  and  based  upon  the  altitude  of 
the  sun  above  the  horizon.  The  climatic  conditions  experienced  within 
these  zones  have  no  such  definite  boundaries.  When  we  come  to  the 
question  of  daily  weather  conditions,  it  is  even  more  difficult  to  assign 
any  fixed  limits  to  areas  of  characteristic  weather  types.  Still,  it  is 
possible  to  designate  a  number  of  zones,  in  the  central  portions  of  which 
the  weather  conditions  are  sharply  marked  off  from  conditions  in  neigh- 
boring zones. 

the  tropical  zoxe. 

The  climatic  belt  designated  as  the  tropical  zone  has  several  sub-zones 
of  characteristic  types  of  weather.  The  entire  zone  is  marked  by  a 
uniformly  high  temperature,  but  the  moisture  conditions  and  atmos- 
pheric movements  vary  greatly  in  neighboring  regions.  The  temperature 
changes  from  day  to  day,  or  from  season  to  season  being  very  small,  the 
seasons  are  marked  by  a  varying  frequency  or  quantity  of  rainfall,  or  by 
a  change  in  the  direction  of  the  wind.  One  day  is  very  much  like  another 
the  year  round,  and  the  weather  cycle  is  the  daily  cycle,  offering  a  strong 
contrast  with  the  rapid  fluctuations  experienced  in  more  northern 
latitudes. 

The  doldrums,  or  equatorial  calms,  are  characterized  by  high  tempera- 
ture and  humidity,  light  winds  or  calms,  much  cloudiness  and  frequent 
and  heavy  rains,  and  almost  daily  thunderstorms — a  combination  caus- 
ing an  oppressive  and  debilitating  atmosphere. 


MARYLAXD    WEATHEK    SERVICE  329 

To  the  north  and  south  of  tlie  doldrums  are  the  trade  wind  belts.  Here 
the  skies  are  mostly  clear,  while  a  fresh,  dry,  northeast  or  south^\"est  wind, 
strongest  over  tlie  ocean,  l)lows  steadily  toward  the  equatorial  belt  of 
highest  mean  temperature. 

Beyond  the  northeast  and  southeast  trades,  there  is  another  belt  of 
light  winds  or  calms,  the  so-called  "horse"  latitudes;  these  are  areas  of 
permanently  high  pressure,  clear  skies  and  warm  dry  air,  resembling  in 
many  respects  the  summer  anti-cyclone  of  the  middle  latitudes.  Within 
this  belt  most  of  the  great  desert  areas  of  the  earth  are  formed,  in  the 
southern  as  well  as  the  northern  hemisphere. 

The  moving  cyclones  and  anti-cyclones,  described  in  preceding  pages 
as  characteristic  of  temperate  zone  weather,  are  conspicuous  by  their  ab- 
sence in  most  portions  of  the  hot  zone.  In  some  portions,  notably  in  the 
West  Indies,  the  Philippines,  and  the  Indian  seas,  cyclones  of  great  inten- 
sity occur  during  the  late  summer  and  early  fall,  the  well-known  hurri- 
canes and  typhoons,  which  are  carried  in  a  westerly  direction  by  the 
general  drift  of  the  atmosphere  in  the  equatorial  regions ;  but  they  are  of 
infrequent  occurrence  when  compared  with  the  constant  succession  of 
temperate  region  cyclones. 

THE    TEMPERATE    ZOXES. 

In  the  middle  latitudes,  north  and  south  of  the  equator  and  extending 
beyond  the  Arctic  and  Antarctic  circles,  the  weather  is  completely  domi- 
nated by  the  moving  cyclones  and  anti-cyclones  described  in  preceding 
paragraphs.  Here  the  daily  monotony  of  tropical  weather  is  replaced  by 
great  variability  of  temperature  conditions  which  mark  the  seasons,  and 
by  the  more  rapid  fluctuations  which  accompany  the  passing  of  cyclones 
and  anti-cyclones.  Tropical  heat  succeeds  polar  cold  and  all  the  weathers 
of  the  globe  are  brought  to  our  doors  during  the  course  of  a  year.  These 
contrasts  become  more  and  more  marked  as  we  approach  the  central  por- 
tions of  the  great  continental  areas  of  the  northern  hemisphere.  In  the 
extreme  nortliwest  of  our  own  country  and  in  the  British  northwest 
territory,  the  breeding  ground  of  cyclones  and  anti-cyclones,  the  contrasts 
in  temperature  experienced  at  a  single  station  within  a  few  hours,  or 


330  THE    CLIMATE    OF   BALTIMORE 

within  very  limited  areas  at  the  same  hour,  are  sometimes  marvelous. 
On  the  10th  of  February  in  the  year  1899,  an  anti-cyclone  developed  over 
Montana  and  the  British  territory  just  beyond.  It  spread  rapidly  over 
the  United  States  as  one  of  the  most  intense  cold  waves  ever  experienced 
in  this  country,,  lowering  the  record  of  intense  cold  in  many  states  in  its 
progress  southeastward  to  the  Gulf  of  Mexico  and  the  Atlantic  coast. 
On  the  morning  of  the  10th  a  minimum  temperature  of  65°  below  zero 
was  registered  in  the  western  part  of  Montana.  Just  west  of  the  moun- 
tains in  the  neighboring  state  of  Washington,  the  temperature  at  the 
same  hour  was  63°  above  zero,  a  difference  of  128°  between  two  points 
along  the  same  parallel  of  latitude  (50°  north)  less  than  300  miles 
apart. 

The  southeastward  progress  across  the  United  States  of  some  of  the 
more  marked  cold  waves  is  frequently  attended  by  strong  inversions  in 
the  normal  distribution  of  temperature  along  the  Atlantic  coast.  The 
front  of  the  cold  wave,  with  its  cold  northwest  winds,  may  reach  Florida 
from  12  to  24  hours  before  it  is  felt  in  New  England  and  the  British 
maritime  provinces,  which  may  be  in  the  center  of  the  well  developed 
cyclone  which  frequently  precedes  the  cold  wave.  Under  such  conditions, 
a  strong  southerly  wind  will  blow  along  the  north  Atlantic  coast  and  raise 
the  temperature  high  above  its  normal  seasonal  value  for  these  coasts, 
while  the  Gulf  states  are  dominated  by  the  intensely  cold  northwest  wind 
of  the  anti-cyclone.  In  such  cases,  temperatures  of  20°  above  zero  or 
less  are  experienced  in  northern  Florida  while  the  warm  southerly  winds 
blowing  over  Newfoundland  raise  the  temperature  to  40°  or  50°  above, 
although  the  latter  region  is  nearly  25°  of  latitude  farther  north  than 
Florida. 

THE   POLAR    ZONES. 

While  we  are  less  familiar  with  the  weather  conditions  of  the  extreme 
north  and  south  portions  of  the  globe  than  with  those  of  the  hot  and 
temperate  zones,  we  have  abundant  evidence  that  within  the  Arctic  and 
Antarctic  the  passing  cyclone  controls  conditions  to  the  highest  latitudes 
attained.    The  fluctuations  in  temperature  are  very  great,  with  changes 


MARYLAND   WEATHER   SERVICE  331 

in  the  direction  of  the  wind,  while  the  cold  is  usually  intense.  While 
there  are  apparently  bright,  clear  and  exhilarating  days,  the  weather  is 
mostly  gloomy  with  a  high  humidity  and  frequent  fogs,  sleet  and  snow. 
The  intense  cold  weakens  the  powers  of  resistance.  With  these  disa- 
greeable weather  conditions  predominating  there  is  the  added  gloom  of 
long-continued  darkness,  the  physiological  effects  of  which  are  exceed- 
ingly distressing. 

THE   SEASONS. 

In  the  middle  latitudes,  and  particularly  over  the  continental  areas, 
the  most  conspicuous  feature  of  the  advance  and  retreat  of  the  seasons 
is  the  marked  rise  or  fall  in  the  mean  temperature  from  month  to  month. 
Take,  for  example,  the  annual  rise  and  fall  of  the  thermometer  at  Balti- 
more as  shown  by  Fig.  25  on  page  111 ;  the  curves  b,  c,  and  d  are  based  on 
the  mean  monthly  maximum,  the  normal  monthly  average  and  the  mean 
monthly  minimum  temperatures  respectively.  The  lowest  temperatures 
occur  in  the  months  of  January  and  February;  from  this  portion  of  the 
curve  there  is  a  steady  rise  at  a  fairly  uniform  rate  to  the  month  of  July, 
followed  by  an  uninterrupted  fall  to  midwinter.  The  smooth,  simple 
curve  represents  a  uniform  increase  and  decrease  in  the  power  of  the  solar 
rays  as  the  sun  increases  and  decreases  in  altitude  in  the  annual  revolu- 
tion of  the  earth  about  the  sun.  This  uniform  increase  and  decrease 
throughout  the  year  is  still  shown  by  constructing  the  annual  curve  from 
the  mean  temperatures  of  successive  five-day  periods.  (See  Table 
XVIII,  page  89.)  If,  however,  we  represent  the  advance  and  retreat  of 
the  seasons  by  curves  based  on  mean  daily  temperatures  in  place  of  mean 
monthly  temperatures,  we  find  a  striking  difference  in  the  character  of 
the  two  sets  of  curves,  as  is  clearly  shown  by  consulting  Plate  III.  There 
is  no  such  uniformity  in  the  daily  progress  of  temperature;  the  serrated 
appearance  of  the  curve  indicates  clearly  that  the  progress  is  marked  by 
successive  temperature  waves  having  an  average  period  of  three  or  four 
days.  The  annual  curve  is  broken  up  into  a  series  of  subsidiary  curves 
of  short  but  irregular  periods,  due  to  the  constant  succession  of  cyclones 
and  anti-cyclones  with  their  accompanying  large  fluctuations  in  tempera- 


332  THE    CLIMATE    OF   BALTIMORE 

ture.  By  substituting  the  actual  temperatures  experienced  upon  each 
day  of  an}'-  given  year,  in  j^lace  of  the  meari  daily  temperatures  for  a 
long  series  of  years,  the  irregularities  of  the  annual  curve  become  enor- 
mously increased.  The  extent  to  which  the  temperatures  experienced 
upon  a  given  day  of  the  year  have  varied  in  past  years  is  shown  in  curves 
A,  C,  and  D  in  Plate  IV.  For  example,  on  the  11th  day  of  February, 
1899,  the  luinimum  temperature  at  Baltimore  was  6°  below  zero;  on  the 
11th  of  February  in  1887,  the  maximum  was  72°  above  zero,  an  extreme 
range  of  78°  for  the  11th  of  February.  Even  in  the  summer  months, 
when  the  variability  in  temperature  conditions  is  least  marked,  the  ex- 
treme ranges  are  about  -10°. 

While  differences  in  temperature  constitute  the  most  conspicuous  fea- 
ture of  the  weather  of  successive  seasons  in  most  portions  of  the  middle 
latitudes,  the  character  and  amount  of  precipitation,  and  the  duration 
and  the  force  of  the  wind  are  factors  of  great  importance,  and  indeed 
these  sometimes  overshadow  the  temperature  changes. 

The  departures  from  the  normal  conditions  of  temperature,  precipi- 
tation and  wind  experienced  in  a  given  season,  must  be  referred  back  to 
the  prevailing  type  of  pressure  distribution  upon  which  they  depend. 
A  clear  knowledge  of  the  relative  distribution  of  pressure  over  a  widely 
extended  area  is  essential  to  a  proper  understanding  of  the  weather 
changes  in  any  given  locality;  this  knowledge  should  extend,  not  only  to 
the  rapidly  moving  cyclones  and  anti-cyclones,  but  also  to  the  larger  areas 
of  high  and  low  pressure  known  as  permanent  cyclones  and  anti-cyclones, 
which  are  a  direct  result  of  the  general  atmospheric  circulation.^ 

As  a  result  of  the  increasing  cold  of  the  winter  months  there  is  formed 
over  the  Xorth  American  continent  a  vast  anti-cyclonic  area.  AVith  the 
passing  of  the  winter,  this  gradually  disappears  to  give  place  to  a  baro- 
metric depression,  or  cyclone.  The  process  is  reversed  over  the  neighbor- 
ing oceans;  here,  while  the  contrasts  between  the  winter  and  sumiuer 
pressure  distribution  are  not  so  marked,  the  pressure  is  lower  in  winter 

^  See:  Teisserence  de  Bort;  Etude  sur  I'hiver  de  1879-80.  Ann.  du  Bureau 
Centr.  Met'L.  Paris,  Vol.  IV,  1881. 


MARYLAND   WEATHER    SERVICE  333 

than  in  summer.  The  changes  in  the  intensity  and  in  the  position  of 
these  great  atmospheric  systems  have  a  direct  influence  upon  the  char- 
acter of  the  seasons  over  the  eastern  portions  of  the  United  States,  and 
especially  in  the  Atlantic  coast  states."  The  best  developed  and  most 
conspicuous  instance  of  this  semi-annual  transfer  of  vast  quantities  of 
air  from  the  continent  to  ocean  during  the  winter  and  from  ocean  to  con- 
tinent during  the  summer,  is  seen  in  the  winter  and  summer  monsoons 
over  India  and  the  Indian  Ocean. 

Bearing  in  mind  what  has  been  said  concerning  the  influence  of  pres- 
sure distribution  on  the  direction  of  winds,  and  hence  on  temperature 
and  precipitation,  we  may  realize  how  variations  from  the  normal  type 
of  pressure  distribution  for  a  given  month  or  season  will  affect  the 
general  character  of  the  weather  of  the  period  in  question.  This  influ- 
ence may  be  graphically  shown  by  calculating  the  mean  monthly  distri- 
bution of  atmospheric  pressure  over  the  North  American  Continent  and 
adjacent  oceans  during  an  abnormally  cold  month  and  an  abnormally 
warm  month,  and  charting  the  results  in  connection  with  a  map  showing 
the  normal  distribution  of  pressure  based  on  a  long  series  of  years  of 
observation.  This  has  been  done  in  succeeding  pages  for  the  months  of 
January,  April,  June,  and  October  as  types  for  the  winter,  spring,  sum- 
mer, and  autumn  seasons  respectively. 

In  the  succeeding  pages  some  of  the  more  conspicuous  types  of  cy- 
clonic and  anti-cyclonic  control  of  the  weather  of  the  Middle  Atlantic 
states  will  be  considered  in  connection  with  a  discussion  of  the  seasons 
in  which  they  most  frequently  occur. 

WINTER  WEATHER. 

The  winter  season  presents  the  most  variable  weather  conditions  of 
the  year.  Practically  every  type  of  weather  may  be  experienced  at  one 
time  or  another  during  the  course  of  the  three  months,  and  sometimes  a 
great  variety  of  types  may  be  crowded  into  the  short  period  of  24  to  48 

*  See:  O.  L.  Fassig.  Types  of  March  Weather  in  the  United  States.  Amer. 
Journ.  Sci.,  New  Haven,  November,  1899,  Vol.  III. 


334  THE    CLIMATE    OF   BALTIMORE 

hours.  A  description  of  winter  weather  conditions  which  prevail  in  the 
vicinity  of  Baltimore  would  include  all  the  types  of  the  year,  though 
some  of  them  attain  their  greatest  development  in  other  seasons. 

An  account  of  weather  conditions  from  day  to  day  in  our  latitudes  is 
mostly  confined  to  a  consideration  of  the  eastwardly  moving  procession 
of  cyclonic  and  anti-cyclonic  systems  across  the  United  States.  While 
for  purposes  of  convenience  and  clearness  our  descriptions  are  confined 
to  well  developed  types,  the  fact  must  not  be  overlooked  that  the  faintly 
developed  systems  are  the  most  frequent  and  consequently  in  the  long 
run  determine  the  general  character  of  the  weather  of  a  given  locality. 

All  of  our  weather  types  may  be  roughly  separated  into  two  fairly 
distinct  classes — (a)  areas  of  unsettled  weather  accompanying  the  pas- 
sage of  cyclones,  or  areas  of  low  pressure — (b)  areas  of  fair  weather  asso- 
ciated with  passing  anti-cyclones,  or  areas  of  high  pressure.  While  it 
is  often  difficult  to  distinguish  these  types  clearly  it  will  be  found  of 
great  convenience  to  adhere  to  the  classification  in  the  following  pages. 

Winter  Cyclones. 

As  stated  in  preceding  paragraphs,  the  weather  of  our  middle  latitudes 
is  characterized  by  an  irregular  succession  of  atmospheric  waves  passing 
from  west  to  east;  the  areas  in  which  the  barometer  reads  high  corres- 
ponding with  the  crest  of  the  waves,  while  the  areas  of  low  barometric 
pressure  may  be  compared  with  the  troughs.  When  these  crests  and 
troughs  are  well  developed  and  sharply  defined  the  latter  are  known  as 
cyclones,  or  simply  as  storms,  while  the  crests  are  called  anti-cyclones; 
in  the  winter  season  when  these  anti-cyclones  develop  to  unusual  inten- 
sity they  constitute  our  cold  waves.  When  they  are  well  developed  and 
move  with  average  speed  across  the  country  these  cylonic  disturbances 
usually  cover  a  period  of  two  to  three  days  in  passing  a  given  meridian. 
As  they  pass  over  a  region  they  bring  to  it  a  fairly  regular  sequence  of 
weather  changes.  The  character  of  these  successive  changes  is  modified 
by  various  conditions.  First  in  importance  is  the  position  of  the  region 
with  reference  to  the  center  of  the  barometric  depression,  or  storm.  The 
path  traversed  by  the  center  of  the  storm  with  reference  to  Baltimore 
depends  largely  upon  the  place  of  its  origin. 


MARYLAND   WEATHER   SERVICE  335 

In  selecting  a  series  of  storms  for  illustration  to  show  the  different 
varieties  of  weather  experienced  in  the  vicinity  of  Baltimore  during  the 
course  of  the  year,  it  will  be  found  convenient  to  classify  them,  basing 
the  classification  upon  the  place  of  origin  of  the  depression,  or,  perhaps 
better,  the  position  of  the  center  of  the  depression  a  day  or  two  before 
its  arrival  over  the  region  about  Baltimore. 

Four  types  will  be  described  in  the  order  of  their  percentage  of 
frequency  across  the  horizon  of  Baltimore — the  Lake  storm,  the  South- 
west storm,  the  Gulf  storm,  and  the  Coast  storm.  These  types  imper- 
ceptibly merge  into  one  another  at  times,  but  they  have  sufficient 
individuality  to  permit  of  ready  separation  into  the  classes  named. 

All  of  these  classes  show  their  most  intense  development  in  the  winter 
season,  with  perhaps  the  exception  of  the  Coast  storm;  the  latter  is 
likely  to  be  the  northward  extension  of  a  West  Indian  hurricane,  and 
hence  shows  a  maximum  frequency  in  the  early  autumn,  or  late  summer. 

THE  LAKE  STORM. 

The  Storm  of  December  24-26,  1902. 
The  daily  weather  map  of  the  United  States  Weather  Bureau  for  8 
a.  m.,  December  23,  1902,  shows  a  distribution  of  pressure  which  caused 
a  fairly  normal  condition  of  winter  temperatures.  A  barometric  pres- 
sure above  the  seasonal  average  prevailed  over  the  eastern  half  of  the 
country  with  a  maximum  over  the  Great  Lakes,  giving  rise  to  northerly 
winds  east  of  the  Mississippi  River.  A  depression,  first  shown  on  the 
map  of  the  22d  over  Puget  Sound,  had  made  its  way  eastward  to  Montana 
and  North  Dakota.  West  of  the  Mississippi  this  depression  had  already 
shown  its  influence  in  a  drift  of  southerly  winds  towards  the  center  of 
depression,  but  the  isotherms  had  not  as  yet  been  greatly  bent  from 
their  normal  trend.  Twenty-four  hours  later,  at  8  a.  m.  of  the  24th, 
the  center  of  the  depression  had  moved  eastward  a  distance  of  about 
600  miles,  its  center  being  over  Lake  Superior.  The  effect  of  24  hours 
of  southerly  winds  in  advance  of  the  center  of  the  storm,  coupled  with 
the  southward  flow  of  winds  from  the  colder  northwest  quadrant  behind 
the  storm  center,  changed  the  isotherms  from  their  normal  east-west 


336 


THE    CLIMATE    OF   BALTIMORE 


Fig.  91.— The  Lake  Storm  of  December  24,  1902. 


29  e 


Fig  92.— The  Lake  Storm  of  December  25,  1902. 


MARYLAND   WEATHER    SERVICE 


337 


trend  to  north-south  lines.  Temperatures  in  ach  ance  of  the  center  were 
raised  15°  to  20°  in  the  southeast  quadrant,  while  there  was  a  fall  of 
equal  amount  in  the  southwest  quadrant.  On  its  way  eastward  the  area 
of  precipitation  grew  in  extent.  The  temperatures  on  all  sides  of  the 
storm  center  being  below  freezing  point  the  precij)itation  was  practically 
all  in  the  form  of  snow.  As  is  most  frequently  the  case,  the  storm 
area  was  oval  in  shape,  with  its  long  axis  extending  north  and  south ; 


Fig.  93.— The  Lake  Storm  of  December  26,  1902. 


as  a  result  the  winds  about  the  center  blew  from  the  southeast  in  advance 
of  the  long  axis,  and  from  the  northwest  in  the  rear  of  the  advancing 
central  line,  on  both  sides  l)lowing  towards  the  trough  of  tlie  lowest  pres- 
sure. 

By  Christmas  morning  the  storm  center  had  moved  eastward  to  the 
Lower  Lake  region,  a  distance  of  about  500  miles,  and  before  the  close 
of  the  day  the  trough  of  lowest  pressure  had  crossed  the  meridian  of 
Baltimore.     The  eastern  edge  of  the  area  of  snowfall  had  reached  the 


338  THE    CLIMATE    OF   BALTIMORE 

Atlantic  coast  by  8  a.  m.,  from  Massachusetts  to  Xorth  Carolina.  Dur- 
ing the  preceding  12  hours  snow  had  fallen  in  varying  amounts  over  the 
entire  area  from  Chicago  eastward  to  the  Atlantic  coast,  and  from  the 
Lakes  southward  to  North  Carolina.  The  area  in  advance  of  the  storm 
over  which  the  temperatures  rose  15°  to  20°  now  extended  to  the  coast, 
while  a  cold  wave  (an  area  of  high  pressure)  followed  close  behind  the 
storm  center,  attended  by  northwesterly  winds  and  clear  skies. 

By  8  a.  m.  of  the  26th  of  December,  the  center  of  the  storm  had 
reached  the  New  England  coast  in  its  due  eastward  progress,  covering 
another  600  miles  in  the  preceding  24  hours.  Here  it  remained  nearly 
stationary  for  24  hours  before  continuing  its  eastward  course  over  the 
North  Atlantic  Ocean,  By  this  time  the  area  of  high  barometric  pres- 
sure following  the  storm  had  spread  over  the  entire  region  from  the 
Eocky  Mountains  eastward  to  the  Atlantic  coast  and  from  the  Lakes  to 
the  Gulf  coast,  carrying  freezing  temperatures  southward  into  Middle 
Florida. 

The  weather  map  of  December  27th  shows  a  condition  which  frequently 
occurs  in  the  winter  months — a  striking  inversion  of  temperatures 
between  Florida  and  Nova  Scotia.  Jacksonville,  Fla.,  had  a  tempera- 
ture of  24°  at  8  a.  m.,  while  Sydney,  N.  S.,  reported  a  temperature 
of  40°  at  the  same  hour.  The  reason  for  this  apparent  anomaly  is 
readily  found  on  examining  the  weather  maps  of  the  preceding  days. 
The  cold  northwest  winds  flowing  out  of  the  area  of  high  pressure  in  the 
rear  of  the  advancing  storm  had  reached  the  Gulf  states  while  the  warm 
southerly  winds  were  still  blowing  over  Nova  Scotia  in  the  southeast 
quadrant  of  the  storm  area. 

The  path  of  this  storm  of  December  24-26,  1902,  from  the  Lake  region 
eastward  is  the  approximate  path  of  nearly  three-fourths  of  the  baro- 
metric depressions  which  exert  a  direct  influence  upon  the  weather  con- 
ditions in  the  vicinity  of  Baltimore.  The  path  of  the  center 
lies  well  to  the  north  of  Baltimore.  The  successive  changes  in  the 
elements  of  the  weather  experienced  during  the  passage  of  this  type  of 
storm  across  the  meridian  of  Baltimore  are  graphically  illustrated  in  the 
accompanying  diagram,  a  brief  description  of  which  will  suffice  to  call 
attention  to  the  most  important  factors.     (See  Figs.  91-94.) 


MARYLAND   "WEATHER   SERVICE 


339 


> 

)- 

\          \    1 

z 

0            s 

h 
Z 

UJ 
Ct 

1 

Q 

\         I   / 

in 

\ 

1 

D 
I 

\ 

UJ 

> 

0 

I 

Q 

UJ 

(r 

a 

/ 

I                 "-    \ 

Z      •' 

7 

/ .^- 

J                      \ 

/                        \ 

K     " 

0 

UJ 

f                              \ 

CO 

Q 

a 

+           r~- 

< 

\                                i 

^                     ^ 
/> 

UJ 

J 
u 

\ 

A      " 

\            / 

00 

\          / 

I 

/ 

IN 

1        / 

U) 

y        •/ 

CD 

^ 

coy 

0)             u 
c\j             w 

^  /^ 

0 

0 

\ 

1 

/     - 

i 

f 

1 

-« lo 

/ 

f3 

V 

\ 

/ 

i 

0 

— v — 

/ 

-« tv 

/ 

r.      ^ 

\ 

/ ' 

/ 

rl 

0 

J 

\ 

V 

/ 

u 

\                        \ 

1 

^ 

V 

>  - 

/ 

0 

0 

\. 

/ 

en 
'I- 

o                \ 

r~ 

/ 

' 

/    - 

►       f- 

►      r^ 

*      2 

C\J 

b 

U 

UJ 

Q 

< 

UJ 

J 
u 

340  THE   CLIMATE    OF   BALTIMORE 

Within  the  horizon  of  Baltimore  the  ajjproach  of  the  storm  from  the 
west  was  announced  by  a  steady  fall  in  the  height  of  the  mercury  in 
the  barometer  after  10  o'clock  in  the  morning  of  the  24th  of  December. 
The  day  began  with  clear  skies;  soon  after  sunrise  the  clouds  began  to 
form,  increasing  in  amount  until  the  sky  became  overcast  by  10  a.  m. 
The  winds  blew  from  the  north  in  the  early  morning.  About  10  a.  m. 
the  direction  changed  to  northeast,  continuing  to  veer  to  east  and  then 
to  soutlieast  and  south  with  the  continued  fall  of  the  barometer.  Co- 
incident with  the  changes  in  the  wind  from  north  to  east  and  southeast 
the  temperature  rose  steadily  until  nearly  midnight;  the  usual  diurnal 
fall  in  temperature  after  3  p.  m.  being  eliminated  by  the  cyclonic  rise. 
The  barometer  continued  to  fall  until  8  a.  m.  of  Christmas  day,  the 
time  at  which  the  trough  of  lowest  pressure  of  the  storm  area  crossed  the 
meridian  of  Baltimore.  At  about  the  same  hour  the  wind  veered  from 
southeast  to  southwest.  The  temperature  continued  to  rise  until  10  a.  m. 
and  then  fell  steadily  with  the  persistent  blowing  of  west  to  noi-thwest, 
winds.  The  humidity  increased  from  40  per  cent  at  10  a.  m.  of  the  24th, 
when  the  winds  changed  to  an  easterly  direction,  to  a  maximum  of  90 
per  cent  at  8  a.  m.  of  the  28th.  With  the  change  of  wind  from  southerly 
to  westerly  the  humidity  fell  rapidly  to  45  per  cent  within  a  period  of 
about  two  hours. 

A  light  dry  snow  began  to  fall  betw^een  10  and  11  p.  m.  of  the  24th, 
with  a  falling  barometer  and  a  southerly  wind.  The  snow  continued 
until  2  a.  m.  of  the  25th,  the  total  amount  lieing  about  three  inches. 
The  sky  remained  overcast  until  10  a.  m.  of  the  25th,  when  the  clouds 
began  to  break  away  soon  after  the  shift  of  the  wind  from  southeast  to 
southwest.  By  8  p.  m.  the  sky  was  clear.  The  usual  sharp  rise  in  pres- 
sure after  the  storm  was  not  experienced  in  the  passage  of  this  depres- 
sion, the  barometer  remaining  comparatively  low  through  the  25th  and 
26th.  As  a  result  there  were  no  high  winds  in  Baltimore  during  the 
progress  of  the  storm :  there  was  a  slight  increase  in  velocity,  however, 
following  the  turn  in  the  barometer  and  the  change  in  direction  of  the 
wind  from  west  to  northwest. 

The  series  of  maps  showdng  the  progress  of  this  storm  across  the 
United  States  well  illustrates  the  normal  winter  conditions  of  a  succes- 


MARYLAND   WEATHER    SERVICE  341 

sion  of  areas  of  high  and  low  pressure  moving  from  west  to  east  across 
the  continent.  On  the  map  of  December  2-ith  we  see  an  area  of  high 
pressure  over  Xew  England,  a  depression  over  the  Upper  Lake  region, 
another  high  area  over  Montana  and  the  Canadian  Xorthwest,  and 
another  depression  appearing  on  the  Pacific  coast  over  Oregon  and 
Washington.  These  systems  move  eastward  at  an  average  rate  of  about 
GOO  miles  per  day  with  their  centers  mostly  between  the  40th  and  50th 
parallels  of  latitude,  bringing  to  localities  over  which  they  pass  their 
characteristic  changes  in  temperature,  in  wind  direction  and  force,  in 
clouds  and  sunshine,  and  in  rainfall  or  snowfall. 

The  Storm  of  January  7-S,  1903. 

The  daily  charts  of  January  6,  7,  and  8,  1903,  issued  by  the  United 
States  Weather  Bureau,  show  the  progress  of  another  storm  of  the  Lake 
region  type.  On  the  5th  a  depression  appeared  upon  the  field  of  the 
map  in  the  extreme  northwest  of  the  Canadian  Provinces.  By  8  a.  m. 
of  the  6th  the  depression  had  crossed  the  boundary  line  into  Xorth  Dakota 
as  a  well  developed  storm,  its  influence  being  felt  over  most  of  the  area 
between  the  Eocky  Mountains  and  the  Mississippi  Valley. 

In  the  succeeding  24  hours  the  center  of  the  storm  had  traversed  a 
distance  of  nearly  a  thousand  miles,  from  Quaj^elle,  Manitoba,  to  Central 
Michigan.  The  depression  developed  no  precipitation  area  until  the 
night  of  the  6th.  By  8  a.  m.  of  the  ith  snow  had  fallen  over  a  sym- 
metrical oval  area  about  the  center,  extending  about  a  thousand  miles 
from  east  to  west  and  about  six  hundred  miles  from  north  to  south. 
Passing  eastward  witli  the  same  rapid  rate  of  progress  the  center  moved 
over  Nova  Scotia  by  8  a.  m.  of  the  following  day.  Wliilc  the  area  of 
snowfall  attending  this  storm  reached  as  far  soutli  as  Tennessee  and 
North  Carolina,  the  precipitation  was  extremely  light  in  Maryland  and 
Virginia,  and  was  of  short  duration.  The  rainfall  or  snowfall  in  Balti- 
more and  vicinity  is  generally  light  and  falls  in  tlic  form  of  brief 
showers  or  snow  flurries  with  storms  of  this  type,  unless  their  centers 
pass  the  meridian  of  Baltimore  within  a  hundred  miles,  or  less,  to  the 
north,  when  the  precipitation  may  be  heavy  and  of  considerable  duration. 


343 


THE    CLIMATE    OF   BALTIMORE 


Fig.  95. — The  Lake  Storm  of  January  7,  1903. 


30.0 


Fig.  96.— The  Lake  Storm  of  January  8,  1903. 


MARYLAND    WEATHER    SERVICE 


343 


23 


344  THE    CLIMATE    OF   BALTIMORE 

In  this  storm  the  normal  trend  of  the  isotherms  was  not  disturbed  to  the 
same  extent  as  in  the  case  of  the  storm  of  December  24-26,  1902,  described 
above.  The  usual  decided  fall  in  temperature  (20  degrees  or  more) 
followed  in  the  path  of  the  storm,  but  in  this  instance  the  cold  wave 
R-as  nearly  24  hours  behind  the  center  of  the  depression  and  did  not 
reach  the  Atlantic  coast.  The  local  changes  during  the  progress  of  the 
storm  were  not  ver}'  pronounced  but  they  were  representative  of  the 
type  following  a  similar  path.  Early  in  the  morning  of  the  7th  Balti- 
more came  within  the  area  of  influence  of  the  Lake  storm  described 
above.  The  barometer  began  to  fall  about  midnight  of  the  6th-7th, 
the  wind  changed  at  the  same  time  from  northwest  to  west  and  soon 
after  to  the  south^^•est ;  by  6  a.  m.  the  wind  had  backed  to  the  south  and 
this  direction  prevailed  until  4  p.  m.  Between  4  and  5  p.  m.  there 
was  an  abrupt  change  of  the  wind  from  south  to  west,  accompanied  by 
a  rise  in  the  barometer.  The  pressure  rose  slowly  though  steadily 
throughout  January  8th  as  the  center  of  the  depression  moved  over  the 
Atlantic  oif  the  New  England  coast.  The  wind  did  not  materially 
increase  in  force  until  2  a.  m,,  about  10  hours  after  the  beginning  of 
the  rise  in  the  barometer,  attaining  a  maximum  velocity  of  28  miles  per 
hour  before  noon. 

The  passage  of  a  coast  storm  on  the  5th-6th  left  a  raw  blustery  Mdnd 
blowing  from  the  west  and  northwest  in  the  afternoon  of  the  6th,  witfi 
clearing  skies.  Cloudiness  increased  during  the  morning  of  the  7th 
upon  the  approach  of  the  Lake  depression  and  the  sky  soon  became 
overcast.  There  was  a  brief  breaking  away  of  the  clouds  between  three 
and  four  p.  m.  Light  snow  fell  between  8.30  and  10.30  a.  m.  of  the 
7th,  between  11.50  a.  m.  and  1.45  p.  m.,  between  4  p.  m.  and  5.30  p.  m., 
and  again  between  9.15  and  9.40  p.  m.  The  total  fall  of  snow  was  not 
much  over  half  an  inch.  During  the  8th  the  sky  was  overcast  with  only 
occasional  brief  intervals  of  sunshine.  There  was  a  slow  and  steady 
fall  in  temperature  and  a  steady  rise  in  the  barometer,  with  brisk  westerly 
winds  in  the  forenoon.  Traces  of  snow  fell  between  11.20  and  11.51 
a.  m.,  12.05  and  12.25  p.  m.,  and  between  7.55  and  8.25  p.  m. 

The  rise  in  temperature  in  advance  of  the  storm  was  not  well  marked. 


MAKYLAXD    WEATHER    SERVICE  345 

This  may  be  readily  accounted  for  by  the  brief  duration  of  tlie  south- 
erly winds  in  advance  of  the  center,  covering  a  period  of  less  than  10 
hours.     (See  Figs.  95-97.) 

The  Storm  of  February  27 -March  1,  1903. 

These  Lake  storms  sometimes  develop  into  disturbances  of  great  extent 
and  intensity.  The  weather  chart  of  8  a.  in.,  February  27,  shows  a  depres- 
sion centered  over  Eastern  Nebraska,  formed  apparently  by  the  union 
of  two  distinct  depressions;  one  of  these  had  its  origin  in  the  Canadian 
Northwest  Provinces,  the  other  in  the  extreme  southwest,  over  Arizona. 
By  8  a.  m.  of  the  27th  a  very  considerable  rain  area  had  already  developed 
over  the  Central  and  Southern  states,  aided  largely  by  the  presence  of  a 
well  developed  area  of  high  barometric  pressure  over  the  Atlantic  Ocean 
oft  the  Middle  Atlantic  states.  During  the  succeeding  24  hours  the 
storm  area  grew  to  unusual  proportions,  while  it  moved  eastward  across 
the  Lake  region  at  a  rate  slightly  above  the  normal  rate  of  progress  for 
such  storms.  By  8  a.  m.  of  the  28th  the  precipitation  area  of  the  pre- 
ceding 12  hours  embraced  all  of  the  country  east  of  the  Mississippi  Elver. 
To  the  South  and  east  of  the  storm  center  the  areas  in  which  southerly 
winds  prevailed,  temperatures  rose  from  15°  to  40°,  and  the  precipitation 
was  in  the  form  of  rain;  M^est  of  the  center  of  the  storm,  in  the  area 
of  northwest  winds,  there  was  a  fall  of  20°  to  30°  in  24  liours.  The 
rain  area  was  not  only  of  unusual  extent,  but  the  eastward  movement 
of  the  storm  was  marked  by  very  heavy  rains,  measuring  an  inch  and  a 
half  to  two  and  a  half  inches  in  24  hours  at  many  stations  in  the  South 
Atlantic  and  Gulf  states.  The  passage  of  the  trough  of  low  pressure 
was  also  the  occasion  for  the  production  of  severe  squalls  and  local 
storms.  By  the  morning  of  March  1  the  storm  center  had  moved  east- 
ward to  the  Gulf  of  St.  Lawrence  followed  closely  by  a  fall  of  20°  to  30° 
over  a  large  area,  embracing  a  dozen  or  more  states. 

The  local  changes  during  the  passage  of  this  wide-spread  storm  across 
the  meridian  of  Baltimore  were  exceptionally  well  marked  and  character- 
istic of  the  well  developed  storm  of  the  type  with  a  path  across  the 
Lake  region  and  down  the  St.  Lawrence  Valley.     (See  Figs.  98  101.) 


THE    CLIMATE    OF    BALTIMOItE 


Fig.  98.— The  Lake  Storm  of  February  27,  1903. 


Fig.  99.— The  Lake  Storm  of  February  28,  1903. 


MARYLAND    WEATHER    SERVICE 


34.7 


On  the  morning  of  the  26th  an  area  of  high  barometric  pressure  rested 
over  the  Atlantic  states,  with  its  center  over  Maryland  and  Virginia. 
The  winds  were  light  and  variahle  in  direction.  The  skies  were  clear, 
resulting  in  heavy  frosts  during  the  preceding  night  and  in  the  early 
morning  hours,  throughout  the  state. 

With  clear  skies  and  light  winds  the  temperature  rose  rapidly  during 
the  day,  and  the  air  became  balmy  and  spring-like.     Tliere  were  a  few 


Fig.  100.— The  Lake  Storm  of  March  1,  1903. 


cirro-stratus  clouds  in  the  early  morning,  but  they  soon  disappeared. 
At  10.40  a.  m.  the  local  Weather  Bureau  Office  received  the  following 
telegram  from  the  Central  Olhce  in  Washington :  "  Southeast  storm 
warnings  ordered  hoisted  along  the  Atlantic  coast  from  ]\Iiami,  Fla., 
to  Charleston,  S.  C.  Storm  over  Texas  is  moving  northeastward.  Brisk 
to  high  easterly  winds  arc  indicated  this  evening  and  tonight  on  the 
South  Atlantic  coast.*' 


348 


THE    CLIMATE    OF   BALTIMORE 


MARYLAND    WEATHER    SERVICE  349 

Clouds  gathered  during  tJie  night,  and  at  dawn  of  the  37th  the  sk}^ 
was  entirely  overcast  with  a  thin  veil  of  stratus  clouds.  The  atmosphere 
was  humid  and  the  clouds  began  to  thicken.  A  solar  corona  was  observed 
in  the  forenoon.  The  barometer  fell  steadily  and  rapidly  throughout 
the  day,  the  wind  changed  to  southeast  and  east,  while  the  temperature 
rose  rapidly  from  a  minimum  of  33°  at  &  a.  m.  to  52°  at  4  p.  m.  A 
light  rain  fell  from  2  p.  m.  to  2.10  p.  m.,  began  again  at  3.35  p.  m., 
continuing  through  the  night.  The  amount  of  rainfall  at  midnight  was 
0.68  inch.  At  11.45  a.  m.  southeast  storm  warnings  were  ordered  up 
along  the  Atlantic  coast  as  far  north  as  Ft.  Monroe,  and  later,  southwest 
storm  warnings  were  ordered  from  Baltimore  to  New  York. 

The  following  day,  February  28,  was  cloudy  and  warmer.  The 
atmosphere  was  humid  and  oppressive  in  the  forenoon,  but  became  more 
pleasant  in  the  afternoon.  Light  fog  formed  during  the  night;  at  dawn 
it  was  dense,  but  soon  became  lighter,  disappearing  by  11  a.  m.  About 
1  p.  m.  there  was  a  temporary  break  in  the  clouds,  but  in  about  an  hour 
a  heavy  stratus  mass  arose  and  rapidly  covered  the  sky.  At  10.30  a.  m. 
storm  warnings  were  ordered  changed  to  northwest  from  South  Carolina 
to  Virginia.  The  rain  which  began  on  the  preceding  day  continued 
to  7  a.  m.,  began  again  about  8.30  a.  m. ;  it  was  heavy  for  a  few  minutes 
after  10  a.  m.  and  continued  wdth  brief  interruptions  until  3.15  p.  m. 
The  total  fall  from  midnight  was  0.46  inch.  The  winds  were  fresh  to 
brisk  between  10  and  11  a.  m.,  increasing  in  the  afternoon  and  evening 
to  high ;  the  maximum  velocity  was  38  miles  from  the  west  at  2.45  p.  m. 

The  barometer  continued  to  fall  rapidly  to  a  minimum  of  29.27  inches 
at  2  p.  m.,  while  the  temperature  rose  steadily  from  52°  at  midnight  to 
71°  at  2  p.  m.  At  this  hour  the  wind  veered  from  south  to  southwest 
and  then  to  the  west,  accompanied  by  a  rapid  rise  in  the  barometer  and  a 
sharp  fall  in  the  temperature.  The  atmosphere  became  crisp  and 
invigorating  throughout  the  balance  of  the  day,  and  the  day  following 
(March  1)  the  barometer  rose  and  the  temperature  fell  rapidly  and 
steadily,  while  the  wind  continued  from  the  west  and  northwest. 

During  the  passage  of  this  storm  the  temperature  rose  38°  in  advance 
of  the  center,  from  a  minimum  of  33°  at  6  a.  m.  of  February  27  to  a 


350  THE    CLIMATE    OF    BALTIMORE 

maximum  of  71°  at  2  p.  m.  of  the  28tli.  Tlie  barometer  fell  an  inch 
during  the  same  period.  After  the  passing  of  the  center  of  the  storm 
across  the  meridian  of  Baltimore  the  temperature  fell  38°  in  30  hours, 
while  the  barometer  rose  over  an  inch  during  the  same  period. 

THE    SOUTHWEST    STORM. 

A  much  frequented  path  for  storms  has  its  origin  in  the  Southwest 
and  trends  northeastward  across  the  Lake  region  and  down  the  St. 
Lawrence  Valley,  or  across  the  New  England  states.  Storms  of  this 
type  may  have  their  origin  in  the  extreme  northwest,  or  they  may  enter  the 
United  States  from  the  Pacific  Ocean  off  the  coast  of  California,  but 
they  dip  far  to  the  south,  their  centers  passing  over  Oklahoma  or  Texas, 
b.efore  proceeding  on  their  way  eastward  by  way  of  the  Lake  region.  In 
their  journey  southeastward  these  storms  gather  energy  and  moisture 
with  increase  in  temperature.  They  are  characterized  by  a  sharp  rise 
in  temperature  in  advance  of  the  center  of  the  depression,  as  the  warm 
moisture  laden  southerly  winds  from  the  Gulf  and  South  Atlantic  are 
drawn  into  the  circulation  for  a  relatively  long  period.  As  they  move 
northward  the  temperature  is  not  only  lowered  by  rising  currents  in 
advance  of  the  storm,  but  also  by  reason  of  their  entrance  into  cooler 
latitudes.  As  a  result  of  the  lowering  of  temperature  and  their  prox- 
imity to  the  main  sources  of  water  supply- — the  Gulf  and  flie  Atlantic 
Ocean — clouds  and  rain  form  rapidly  over  a  very  large  area  about  their 
centers.  While  the  paths  of  such  storms  may  not  pass  in  closer  proximity 
to  Baltimore  than  do  the  Northwest  Lake  storms,  their  rain  areas  extend 
farther  southward  and  eastward  from  their  centers  and  hence  bring  to 
Baltimore  a  longer  period  of  unsettled  weather  and  a  heavier  rainfall. 

The  Storm  of  Fchniary  3-5,  1903. 

This  storm  entered  the  United  States  from  the  Pacific  Ocean  on 
February  1.  At  8  a.  m.  of  the  2d  its  center  was  over  Arizona,  and  on 
the  3d  over  Texas.  During  the  succeeding  24  hours  the  storm  turned 
sharply  to  the  northeast,  increasing  in  energy  and  area,  reaching  Lake 
Michigan  by  8  a.  m.  of  the  -ith.     By  this  time  the  rain  area  had  already 


MARYLAND   WEATHER   SERVICE 


351 


reached  the   Atlantic  coast  from   Florida  to  Maine,  while  it  extended 
westward  to  ^Tebraska. 

In  its  southeast  quadrant  the  temperature  rose  20°  or  more  in  24 
hours,  while  a  marked  cold  wave  closely  followed  the  center  of  the  depres- 
sion to  the  southwest,  with  a  fall  of  20°  to  40°  in  24  hours.  In  advance 
of  the  storm  the  precipitation  occurred  as  rain,  excepting  in  the  north- 
east where  the  temperature  fell  below  32°.     Here,  and  to  the  west  of  the 


Fig.  102.— The  Southwest  Storm  of  February  3,  1903. 


storm  center,  snow  fell  over  a  large  area,  in  many  places  to  a  great 
depth.  The  barometric  gi'adients  in  this  storm  were  very  steep,  the 
difference  between  tlic  pressure  at  the  center  and  the  outer  edge  of  the 
storm  being  an  inch  or  more.     (See  Figs.  102-10.").) 

Tiie  following  description  of  the  conditions  at  Baltimore  during  the 
passage  of  this  storm  is  taken  from  the  records  of  the  local  ullice  of 
tlie  United  States  Weather  lUireau  : 


352 


THE   CLIMATE   OF   BALTIMORE 


Fig.  103.— The  Southwest  Storm  of  February  4,  190:; 


Fig.  104.— The  Southwest  Storm  of  February  5,  1903. 


MARYLAND   WEATHER    SERVICE 


353 


February  3,  1903.  A  warm  cloudy  day.  Cirrus  clouds  formed  rapidly  after 
7  a.  m.,  the  sky  becoming  overcast  by  10  a.  m.  The  clouds  increased  in 
density.  Light  rain  began  at  10.55  p.  m.  and  continued  into  the  night.  At 
midnight  the  amount  of  precipitation  was  0.05  inch.  The  atmosphere  was 
balmy  and  springlike. 


Fig.  105.— The  Southwest  Storm  of  February  3-6,  1903. 


February  //,  1903.  The  day  continued  cloudy  and  warm  with  a  sultry 
atmosphere  in  the  morning.  The  temperature  rose  to  66°  at  4  p.  m.,  then  fell 
sharply  7°  just  before  5  p.  m.,  followed  by  a  steady  fall.  The  sky  was  over- 
cast until  1.50  p.  ra.  The  strato-cumulus  clouds  changed  to  cumulus  by  2.30 
p.  m.;  these  in  turn  disappearing  by  4  p.  m.     Dense  fog  prevailed  during  the 


354  THE    CLIMATE    OF   BALTIMORE 

preceding  night,  became  light  in  the  early  morning,  and  disappeared  by  10.30 
a.  m.  The  light  rain  of  the  night  before  continued  into  the  morning,  be- 
coming heavy  about  6.30  a.  m.,  0.20  inch  falling  in  5  minutes.  This  brief 
downpour  was  preceded  by  a  single  flash  of  lightning.  The  rain  continued  at 
intervals  until  8.30  a.  m.  The  total  fall  from  midnight  was  0.86  inch.  A 
slight  peal  of  thunder  was  heard  at  10.21  a.  m.  At  4.50  p.  m.  there  appeared 
an  inky-black,  closely  compacted  mass  of  strato-cumulus  clouds,  driven  from 
the  northwest,  though  the  cloud  mass  showed  a  distinct  northeastward  move- 
ment. By  5  p.  m.  the  entire  mass  had  risen  above  the  western  horizon, 
covering  about  six-tenths  of  the  sky.  On  the  northern  edge  of  the  cloud 
mass  several  cumulo-nimbus  of  the  "  anvil "  variety  were  seen.  Rising  above 
the  western  horizon  were  cumuli,  small  in  size,  and  extending  north  and 
south  for  about  25°,  with  an  overlying  cirro-stratus  layer.  There  were  three 
air  currents:  The  upper  current  was  moving  from  the  west;  the  middle 
current  from  the  southwest;  the  surface  wind  was  from  the  northwest. 
Though  the  cloud  mass  moved  eastward  in  a  body,  the  northeast  end  seemed 
fixed,  and  a  general  commotion  was  noticed  in  the  base  of  the  cloud  strata 
in  this  portion;  mammo-cumulus  clouds  appeared  and  disappeared  for  about 
ten  minutes.  At  5.15  p.  m.  breaks  occurred  in  the  mass,  exposing  snow-white 
cumulus  peaks  with  the  crowns  growing  in  size,  indicating  ascending  air 
currents.  At  5.30  p.  m.  the  mass  was  steadily  being  pushed  southeastward 
and  an  alto-stratus  layer  set  in  from  the  northwest.  The  western  edge  of  the 
cloud  mass  passed  over  the  station  at  5.40  p.  m.  From  this  time  until  6.50 
p.  m.  the  mass  was  very  dark  in  color,  except  on  the  extreme  northeast  edge, 
where  several  snow-white  mountainous  cumulo-nimbus  prevailed.  From  and 
among  these  cumulo-nimbus  broad  flashes  of  lightning  were  seen  from  5.50 
p.  m.  until  6.50  p.  m.  Two  successive  peals  of  thunder  were  faintly  heard 
in  the  eastern  suburbs  of  the  city  at  5.30  p.  m.  Southwest  storm  warnings 
were  ordered  up  along  the  coast  from  "Wilmington,  N.  C,  to  New  York.  The 
winds  became  brisk  to  high  after  7  p.  m.,  with  a  maximum  velocity  of  38 
miles  from  the  west  at  7.15  p.  m. 

Fehruary  5,  J903.  The  day  was  partly  cloudy  and  colder.  The  sky  was 
overcast  during  the  forenoon;  clouds  began  to  break  away  about  noon,  and 
by  3.30  p.  m.  they  had  disappeared.  Brisk  to  high  westerly  winds  con- 
tinued throughout  the  night  and  during  the  day,  decreasing  to  fresh  in  the 
evening;  the  maximum  velocities  exceeded  40  miles  an  hour.  Considerable 
damage  was  done  by  the  wind  to  signs  and  awnings,  and  a  few  houses  were 
partially  unroofed.  Snow  flurries  occurred  between  8  a.  m.  and  11  a.  m., 
but  no  snow  remained  on  the  ground;  the  total  fall  was  less  than  a  tenth  of 
an  inch. 


The  Storm  of  December  26-28,  190Jf. 
This  disturbance,  like  the  storm  described  in  the  preceding  paragraphs, 
had  its  origin  over  the  Pacific  Ocean.     Its  center  appeared  off  the  coast 
of  Oregon  on  the  24th  inst.     Moving  rapidly  southeastward  across  the 


MARYLAND    WEATHER  ■  SERVICE  355 

Eocky  Mountains  the  storm  reached  Texas  on  the  morning  of  the  26th; 
recurving  sharply  northeastward  the  center  was  over  Central  Illinois  24 
hours  later  and  over  Toronto  on  the  morning  of  the  28th.  Following 
the  usual  course  down  the  St.  Lawrence  Valley  the  storm  passed  eastward 
over  Labrador  to  the  Atlantic,  crossing  the  continent  from  ocean  to  ocean 
in  just  five  days  along  a  path  about  -iOOO  miles  in  length.  Assuming 
a  uniform  rate  of  speed  the  average  daily  movement  was  800  miles. 
The  rate  varied  from  1000  miles  in  24  hours  from  the  Pacific  Ocean 
across  the  Eocky  Mountains,  to  500  miles  in  crossing  the  Lake  region. 

The  storm  was  characterized  by  a  precipitation  area  of  unusual  extent, 
and  by  heavy  local  rains  and  snows.  The  isotherms  were  bent  from  their 
normal  east-west  direction  to  a  north-south  trend  near  the  center  by  the 
warm  southerly  winds  in  advance  of,  and  the  cold  northwest  winds  in 
the  rear  of,  the  center  of  the  storm.  The  rise  in  temperature  in  the 
southeast  quadrant  was  20°  to  30°,  while  the  subsequent  fall  in  the 
southwest  quadrant  varied  from  20°   to  50°  in  24  hours. 

The  local  changes  in  Baltimore  during  the  progress  of  this  storm 
were  well  marked  and  characteristic.  While  the  center  of  the  storm 
was  over  Texas,  on  the  morning  of  the  26th,  an  area  of  high  barometric 
pressure  rested  over  the  Xew  England  states.  Tliis  distribution  of 
pressure  caused  north  to  northeast  winds  in  the  Middle  Atlantic  states. 
As  the  storm  moved  eastward  and  northward  toward  the  Lake  region  the 
wind  at  Baltimore  veered  to  east  and  southeast,  and  by  noon  of  the 
27th  it  had  become  south.  During  the  night  of  the  27th-28th,  while 
the  center  of  the  storm  was  over  the  Lake  region,  a  secondary  depression 
developed  in  the  southeast  quadrant  of  the  main  storm,  over  eastern 
Pennsylvania  and  Xew  York,  causing  a  sudden  change  of  Avind  to  north 
at  Baltimore;  as  the  storm  center  moved  eastward  the  winds  settled  to 
northwest  witli  rapidly  increasing  velocity. 

The  barometer  fell  from  30.24  inches  at  8  a.  m.  of  the  26th  to  29.10 
inches  at  4  a.  m.  of  the  28th,  and  rose  again  to  30.00  inches  by  10  a. 
m.  of  the  29tii.  Co-incident  witli  tlie  fall  in  pressure  and  the  changes 
in  the  direction  of  the  wind  to  the  south,  noted  above,  the  temperature 
rose  from  26°  at  6  a.  m.  of  the  26th  to  55°  at  4  a.  m.  of  the  28th,  then 


356 


THE    CLIMATE   OF   BALTIMORE 


Fig.  106.— The  Southwest  Storm  of  December  26,  1904. 


Fig.  107.— The  Southwest  Storm  of  December  27,  1904. 


MARYLAND   WEATHER   SERVICE 


357 


fell  with  change  of  wind  to  the  north  and  northwest,  to  20°  at  6  a.  m. 
of  the  29th.  This  cyclonic  rise  and  fall  in  temperature  totally  obliterated 
the  diurnal  fluctuation  usually  noted  in  the  daily  temperature  curve. 
The  maximum  temperature  occurred  at  4  a.  m.  of  the  28th.  There  was 
a  steady  rise  during  the  27th  from  midnight  to  midnight,  and  a  steady 
and  regular  fall  throughout  the  following  day. 

The  rain  was  continuous  but  light.     Beginning  at  9  a.  m.  of  the  26th 
as  a  light  misting  rain  it  continued  as  such  without  interruption  until 


Fig.  108.— The  Southwest  Storm  of  December  28,  1904. 


10.25  p.  m.,  when  it  became  heavier.  About  6  a.  m.  of  the  27th  it  again 
changed  to  a  light  mist  which  continued  to  the  end  of  the  precipitation 
period  between  4  and  5  p.  m.  The  total  amount  of  rainfall  (including 
some  sleet)  fdr  the  32  hours  was  only  0.44  inch.     (See  Figs.  106-109.) 

The  daily  journal  of  the  local  Weather  Bureau   Office  contains  the 
following  remarks  concerning  conditions  on  the  27th  and  28th: 

December  27,  190 Jf.     A  cloudy  day.     Continuous  fog.     Light  rain,  continu- 
ing from  midnight  yesterday,  turned  to  misting  rain  at  6.05  a.  m..  and  ended 


358 


THE    CLIMATE    OF   BALTIMORE 


MARYLAND   WEATHER    SERVICE  359 

at  4.30  p.  m.  Southwest  storm  warnings  were  ordered  up  by  the  Chief  of 
Bureau  at  9.45  a.  m.  from  Jacksonville,  Fla.,  to  Fort  Monroe,  and  southeast 
warnings  at  11.15  a.  m.  from  Baltimore  to  New  York.  A  cold  wave  warning 
was  received  at  9.55  p.  m. 

Dece7nber  2S,  190.'i.  Partly  cloudy  until  9  a.  m.,  followed  by  cloudy;  clear 
after  1.30  p.  m.  A  slow  steady  rise  in  temperature  since  Christmas  morning 
culminated  in  a  sharp  rise  to  55°  at  4  a.  m.  From  this  hour  the  temperature 
fell  steadily  to  22°  at  midnight.  Light  rain  fell  between  1.15  a.  m.  and  2.10 
a.  m.,  amount  0.01  inch.  The  wind  became  brisk  at  9.15  a.  m.  and  continued 
so  until  nearly  midnight.  The  velocity  rose  to  a  maximum  of  40  miles  per 
hour  from  the  west  at  12.45  p.  m. 

The  Storm  of  December  12-13,  1903. 

This  storm  first  appeared  within  the  field  of  view  ou  the  lUth  of 
December  in  the  extreme  Nortliwest.  It  crossed  the  Eoclcy  Mountain 
range  in  Montana  in  a  southeast  course  during  the  night  of  the  lOth-llth, 
and  its  center  was  over  Missouri  and  Arkansas  at  8  a.  m.  of  the  12th. 
Here  it  recurved  to  the  uortlieast,  taking  the  usual  course  across  the 
Lake  region,  down  the  St.  Lawrence  Vallev  and  over  Labrador,  where  it 
disappeared  beyond  the  field  of  the  weather  map  on  the  14th  inst. 
This  storm  resembled  the  southwest  storm  described  above  in  most 
respects.  There  was  an  important  dift'erence,  however,  in  the  form  of 
the  system  of  isobars  surrounding  the  center  as  the  storm  crossed  the 
meridian  of  Baltimore  on  December  13.  The  (jval  shape  of  isobars,  with 
the  long  axis  extending  approximately  north  and  south  is  characteristic 
of  many  of  this  class  of  storms.  The  change  from  easterly  to  westerly 
winds,  as  the  trough  of  low  l)aro meter  moves  eastward,  is  very  abrupt, 
and  is  frequently  attended  by  severe  squalls  or  thunderstorms.  The 
isotherms  extend  nearly  north  and  south  and  are  close  together  in  the 
vicinity  of  the  trough  of  low  pressure,  or,  in  other  words,  the  tempera- 
ture gradient  is  very  steep  and  contrasts  are  great.  In  the  case  of  this 
particular  storm  of  the  13th  there  was  a  difference  of  50°  at  8  a.  m. 
between  Baltimore  and  Tiidinimpolis.  on  the  same  ])arallol  of  latitude. 
or  a  difference  of  50°  between  the  southerly  winds  prevailing  in  advance 
of  the  center  and  the  northwest  winds  which  blew  out  of  the  w^ell 
developed  area  of  high  pressure  in  the  rear  of  the  storm. 

The  trough  of  low  pressure  passed  over  Baltimore  almost  at  the 
exact  time  of  the  8  a.  m.  observaiions  of  the  Unilcd   States  Weather 

24 


360 


THE    CLIMATE    OF    BALTIMORE 


LOW 


LOW 


Fig.  110.— The  Southwest  Storm  of  December  12,  1903 


LOW\         !^o\> 


Fig.  111.— The  Southwest  Storm  of  December  13,  1903. 


MARYLAND   WEATHER    SERVICE 


361 


362 


THE    CLIMATE    OF    BALTIMORE 


Bureau.  A  detailed  presentation  of  the  successive  local  changes  during 
the  13th,  as  this  storm  traversed  the  horizon  of  Baltimore  will  Ije  found 
in  the  accompanying  diagram.  It  is  also  possible  in  this  case  to  show 
the  hourly  changes  in  the  relative  humidity.  At  8  a.  m.,  with  change 
of  wind  from  south  to  southwest  and  then  to  northwest,  there  was  a 
remarkably  rapid  change  in  the  humidity,  the  decrease  amounting  to 
about  55  per  cent  in  four  hours.     (See  Figs.  110-112.) 


Fig.  113. — Paths  and  Rain  Areas  of  Southwest  Storms  of  January,  1898. 


In  the  reports  of  the  local  office  of  the  United  States  Weather  Bureau 
the  loth  is  described  as  cloudy  in  the  forenoon  and  clear  in  the  afternoon. 
The  temperature  was  very  high  in  the  morning,  with  a  maximum  of  52° 
about  8.30  o'clock.  With  a  sudden  change  of  wind  at  this  hour  from 
southwest,  through  the  west,  to  northwest,  a  rapid  fall  in  temperature 
took  place  (10°  in  the  first  hour),  and  it  continued  to  grow  colder  to  a 
minimum  of  30°  at  midnight.     The  atmosphere  was  crisp  and  invigorat- 


MARYLAND    WEATHER    SERVICE  363 

ing  in  the  afternoon.  A  blustering  wind  prevailed  in  the  forenoon. 
Light  rain  began  during  the  night  or  early  morning,  and  ended  at  9.30 
a.  m.  The  total  amount  was  0.30  inch.  The  wind  became  brisk  shortly 
after  9  a.  m.,  changing  to  high  westerly  winds,  and  then  to  northAvest, 
with  a  maximum  velocity  of  41  miles  per  hour  at  9.30  a.  m.  A  cold 
wave  warning  was  received  at  noon,  announcing  a  probable  change  of 
20°  to  30°  before  the  close  of  the  following  day.  Southwest  storm 
warnings  had  been  ordered  up  along  the  coast  from  Savannah  to  New 
York  on  the  12th;  tlVose  were  changed  to  northwest  on  the  morning  of 
the  13th. 

These  southwest  storms  are  usually  accompanied  by  large  rain  areas 
and  heavy  local  rains.  At  times  a  series  of  these  storms  will  follow  one 
another  in  close  succession,  all  taking  approximately  the  same  path, 
from  Texas  across  the  Lake  region  and  Xew  England,  or  the  St.  Lawrence 
Valley  out  into  the  Atlantic.  A  remarkable  series  of  tliis  kind  was 
experienced  during  the  month  of  January,  1898.  The  .accompanying 
chart  (Fig.  113)  shows  the  paths  of  six  storms  of  this  type  all  occurring 
between  the  8th  and  26th  of  January,  1898,  together  with  the  total 
amount  and  distribution  of  precipitation  recorded  along  the  various  paths. 
The  rate  of  movement  of  the  storms  is  shown  by  the  circles  along  the 
lines  illustrating  the  storm  paths,  the  intervals  representing  periods  of 
twelve  hours. 

THE    GULF    STORM. 

Many  of  the  storms  which  have  their  origin  in  the  southwest  or  over 
the  Pacific,  and  cross  tlie  country  along  the  southwest  path,  continue 
their  southeast  course  to  the  Gulf  before  recurving  to  the  northeast. 
Some  have  their  origin  over  the  Gulf  of  Mexico  and  move  northeastward 
to  tlie  Gulf  of  St.  Lawrence.  The  path  taken  by  these  storms  brings 
tlicir  centers  very  close  to  Baltimore.  Sometimes  the  center  of  the 
barometric  depression  passes  just  to  the  west  of  Baltimore,  sometimes 
to  the  east,  and  occasionally  immediately  over  the  city.  They  are  usually 
accompanied  by  heavy  precipitation,  and  by  high  winds  along  the  coast. 
Very  frequently  these  storms  develop  over  the  Gulf  of  ]\Iexico  while 


364  THE    CLIMATE    OF   BALTIMORE 

an  area  of  high  pressure  prevails  over  the  New  England  states.  Under 
the  influence  of  this  distribution  of  pressure,  northeast  to  east  winds 
set  in  over  the  Middle  Atlantic  states  and  southeast  winds  over  the 
South  Atlantic  states.  The  rain  area  spreads  rapidly  northward  and 
eastward  under  these  conditions  and  reaches  Baltimore  while  the  center 
of  the  depression  is  still  in  the  Gulf  states. 

The  average  winter  temperature  of  Baltimore  is  close  to  the  freezing 
point;  hence  slight  changes  in  temperature  will  change  the  form  of 
precipitation  from  rain  to  snow  or  from  snow  to  rain,  or  to  the  disagree- 
able intermediate  stage  of  sleet.  As  these  Gulf  storms  are  nearly  always 
preceded  by  comparatively  high  temperatures  and  followed  by  tempera- 
tures below  the  freezing  point,  they  are  apt  to  cause  much  personal  dis- 
comfort, with  their  rain,  sleet,  and  snow,  resulting  in  slushy  or  icy 
streets  in  the  cities.  Farther  north  the  precipitation  is  mostly  in  the 
form  of  snow,  and  a  short  distance  to  the  south  it  is  all  rain.  The  high 
winds  which  frequently  accompany  this  type  of  storm  not  only  increase 
the  discomfort  but  add  an  element  of  danger. 

The  Storm  of  February  1-3,  1902. 
{Center  passes  west  of  Baltimore.) 

The  weather  map  of  S  a.  m.,  February  1,  1902,  shows  the  prevalence 
of  two  well  developed  areas  of  high  barometric  pressure,  one  in  the  north- 
east, with  its  center  over  the  Gulf  of  St.  Lawrence,  the  other  in  the 
extreme  northwest,  centered  over  Idaho  and  Montana.  In  the  Gulf 
states  and  in  the  Southwest  the  barometer  was  low,  and  unsettled  weather 
prevailed  from  the  Lake  region  to  the  Gulf,  and  from  the  Atlantic  coast 
westward  nearly  to  the  Rocky  Mountains.  In  the  Atlantic  coast  states 
the  barometric  depression  was  already  well  developed,  and  rain  was  fall- 
ing at  8  a.  m.  throughout  the  South  Atlantic  states,  in  Virginia,  Mary- 
land, and  Pennsylvania,  and  snow  in  New  York  and  the  New  England 
states. 

As  the  storm  moved  rapidly  northeastward  it  developed  in  intensity 
and  in  definiteness  of  outline,  the  rains  became  heavier  and  the  area 
of  precipitation  increased.     The  high  area  over  the  Gulf  of  St.  Lawrence 


MARYLAND   WEATHER    SERVICE 


365 


.30.4 


Fio.  114— The  Gulf  Storm  of  February  1,  1902. 


Fig.  115.— The  Gulf  Storm  of  February  2,  1902. 


366 


THE    CLIMATE    OF   BALTIMORE 


remained  stationary  while  that  in  tiie  extreme  Northwest  moved  rapidly 
southeastward  accompanied  by  a  decided  fall  in  temperature  in  the 
southwest  quadrant  of  the  storm  area.  At  8  a.  m.  of  the  2d  of  February 
the  area  of  lowest  barometer  was  over  Pennsylvania,  the  center  of  the 
storm  having  passed  just  to  the  west  of  Maryland  during  the  preceding 
nisrht.  The  center  of  the  western  hi2:h  area  was  over  Kansas  and  Okla- 
homa.     During  the  preceding  24  hours  a  fall  of  15°  to  30^  in  tempera- 


FiG.  116.— The  Gulf  Storm  of  February  3,  1902. 


ture  was  experienced  over  a  wide  area  from  Iowa  and  Nebraska  southward 
to  the  Gulf  coast.  High  easterly  winds  prevailed  during  the  night  and 
early  morning  along  the  coast  from  the  South  Atlantic  to  the  New 
England  states.     (See  Figs.  114-117.) 

By  the  morning  of  the  3d  the  storm  center  had  moved  to  tlie  New 
England  states,  the  cold  wave  had  reached  the  Atlantic  coast  from 
Florida  to  North  Carolina,  and  had  overspread  most  of  Virginia,  Mary- 
land,  and  Pennsylvania.     The  local  changes  at  Baltimore   during  the 


:makylaxd  weather  service 


367 


368  THE    CLIMATE    OF   BALTIMORE 

passage  of  this  storm  are  indicated  in  the  accompanying  diagram,  and  in 
the  following  extracts  from  the  daily  journal  of  the  Weather  Bureau : 

February  1,  1902.  On  February  1,  while  the  center  of  the  storm  was  over 
the  Gulf  States,  the  day  was  cloudy,  the  sky  being  continuously  overcast. 
At  7.45  a.  m.  precipitation  began  in  the  form  of  sleet,  turning  in  10  minutes 
to  a  light  misting  rain.  The  winds  were  from  northeast  to  north  from  noon 
to  midnight,  and  very  light,  averaging  but  3  to  4  miles  per  hour.  Light  rains 
continued  at  intervals  until  8.40  p.  m.,  the  entire  amount  for  the  day  being 
but  0.07  inch.  The  day  was  disagreeable;  the  sidewalks  were  icy.  The 
maximum  temperature  of  the  day  was  37°  at  6  p.  m.  The  barometer  fell 
steadily  throughout  the  day  from  30.03  inches  at  4  a.  m.  to  29.77  inches  at 
midnight.     The  relative  humidity  was  approximately  100  per  cent  all  day. 

February  2,  1902.  The  day  continued  cloudy  during  the  forenoon.  The 
temperature  rose  slowly  to  39°  at  2  p.  m.,  while  the  barometer  fell  to  29.25 
inches.  The  wind  changed  from  north  to  east  at  8  a.  m.  and  to  west  at  10 
a.  m.,  and  continued  light  in  force.  Early  in  the  afternoon  the  wind  began 
to  increase  in  force,  reaching  a  maximum  of  33  miles  per  hour  from  the  west 
between  4  and  5  p.  m.,  shortly  after  the  barometer  began  to  rise.  From  2 
p.  m.  the  temperature  fell  steadily  to  15°  at  midnight  of  the  following  day. 
After  an  interval  of  several  hours  light  rain  began  again  between  2  a.  m.  and 
3  a.  m.  and  continued  without  interruption  until  about  noon,  becoming  heavy 
at  'times.  From  noon  to  1.40  p.  m.  the  precipitation  was  a  mixture  of  rain 
and  snow,  the  snow  melting  as  it  fell.  The  total  fall  of  rain  and  snow  com- 
bined was  0.44  inch. 

The  day  as  a  whole  was  extremely  disagreeable.  The  sidewalks  were  icy 
and  dangerous  to  pedestrians.  In  the  forenoon  the  gutters  and  streets  were 
filled  with  slush,  which,  as  night  approached,  became  frozen  solid.  Some 
damage  was  done  to  awnings,  signboards,  and  chimneys  by  high  winds.  The 
wind,  however,  cleared  the  harbor  of  floating  ice.  Light  fog  prevailed  during 
the  preceding  night  and  lifted  at  about  11  a.  m.  Northwest  storm  warnings 
were  ordered  up  at  10  a.  m.  from  Florida  to  Baltimore.  A  cold  wave  warning 
was  received  at  2.50  p.  m.,  forecasting  a  fall  to  15°,  or  below,  in  the  interior 
of  the  State,  and  to  20°  along  the  coast.  The  clouds  disappeared  rapidly  after 
3  p.  m.  and  by  5  p.  m.  the  sky  was  clear. 

February  3,  1902.  The  day  was  clear  and  much  colder  than  the  2d.  The 
temperature  fell  to  15°  at  9  a.  m.  There  "were  no  clouds  excepting  a  few 
small  cumuli  in  the  afternoon.  The  wind  continued  brisk  during  the  night, 
but  diminished  towards  noon.  Navigation  was  free  on  the  western  side  of 
the  Bay,  but  along  the  eastern  shore  the  ice  was  piled  up  by  the  winds.  All 
tributaries  of  the  Chesapeake  were  frozen  solid. 

The  Storm  of  January  5-1 ,  1905. 
(Center  passes  over  Baltimore.) 
On  the  morning  of  January  5,  1905,  a  somewhat  similar  distribution 
of  pressure  obtained  to  tliat  of  February  1.  1902,  described  above.     An 


MARYLAND   WEATHER    SERVICE 


369 


Fig.  118.— The  Gulf  Storm  of  Jauuary  5,  1905. 


Fio.   119.— The  Gulf   Storm   of  .lanuary   6,   1005. 


370 


THE    CLIMATE    OF    BALTIMORE 


area  of  liigh  barometer  prevailed  over  the  Atlantic  coast  states,  and 
another  over  the  Rock}-  ]\[ountain  Plateau.  The  pressure  was  low  over 
the  Mississippi  Valley  with  a  tendency  to  deepen  over  the  Gulf  states. 
By  8  a.  m.  of  the  following  day  the  Atlantic  coast  area  of  high  pressure 
had  concentrated  over  the  Xew  England  states,  while  the  Eocky  ^lountain 
high  area  had  changed  but  little  in  intensity  or  outline.  The  center 
of  the  barometric  depression  had  been  transferred  to  Xorthern  Floi'ida 


^. 


% 


-36 


Fig.  120.— The  Gulf  Storm  of  January  7,  1905. 


and  Southern  Georgia.  This  combination  of  pressure  along  the  Atlantic 
coast  always  gives  rise  to  northeasterly  winds  with  a  steady  rain  or 
snow.  At  8  a.  m.  of  the  Gth  rain  was  falling  in  the  South  Atlantic 
states  and  snow  in  the  Middle  Atlantic  and  Xew  England  states.  The 
snow  area  also  reached  westward  to  the  Ohio  Valley  and  the  Lake  region 
in  connection  with  the  development  of  a  secondary  depression  over  Lake 
Michigan.     (See  Figs.  118-121.) 


MARYLAND   WEATHER    SERVICE 


371 


372  THE    CLIMATE    OF   BALTIMORE 

After  reaching  the  coast  the  storm  took  a  sharp  turn  northward, 
increasing  in  intensity  as  it  followed  the  coast  line.  The  center  passed 
directly  over  Baltimore  at  about  4  a.  m.  of  the  7th  with  an  abrupt  change 
in  the  direction  of  the  wind  from  south  to  northwest,  and  a  fall  in 
temperature.  The  winds  were  light  to  fresh  during  the  progress  of  the 
storm  over  Baltimore,  only  exceeding  20  miles  per  hour  for  a  short 
time  between  3  p.  m.  and  4  p.  m.  By  the  morning  of  the  8th  the  center 
had  passed  northward  to  the  Lower  St.  Lawrence  River. 

The  temperature  rose  rapidly  20°  to  40°  along  the  Atlantic  coast 
in  advance  of  the  center  of  the  storm,  but  fell  more  slowly  after  the 
center  had  passed,  a^?  the  high  area  of  the  Eocky  Mountain  region  was 
advancing  but  slowly  eastward  behind  the  storm.  Heavy  rains  marked 
the  spread  of  the  storm  in  its  eastern  half  all  along  the  Atlantic  coast; 
rains  of  one  to  two  inches  in  24  hours  were  reported  from  many  of  the 
Weather  Bureau  stations.  The  precipitation  at  Baltimore  amounted 
to  2.34  inches  during  the  12  hours  from  noon  to  midnight  of  the  6th. 

Some  details  of  the  local  conditions  at  Baltimore  are  shown  in  the 
following  extracts  from  the  daily  journal  of  the  local  office  of  the  United 
States  Weather  Bureau : 

January  5,  1905.  A  cold  cloudy  day.  Light  snow  began  at  8.30  a.  m.  and 
ended  at  10  a.  m.  The  winds  were  westerly  in  the  forenoon  and  easterly  in 
the  afternoon. 

January  6,  1905.     The  day  was  cloudy  and  somewhat  warmer  than  yester- 
day.    Light  rain  began  at  8.55  a.  m.,  ended   at  9.10  a.   m. ;    began  again  at 
12.10  p.  m.  and  continued  to  midnight.     The  rain  was  heavy  from  7.40  p.  m.  to 
7.49  p.  m.,  0.22  inch  falling  within  the  9  minutes.     The  total  precipitation  for' 
the  day  was  2.34  inches. 

January  7,  1905.  A  cloudy  day  until  6.30  p.  m.;  the  clouds  broke  away  soon 
after  and  by  8  p.  m.  the  sky  was  clear  and  remained  so  until  midnight.  The 
rain  of  the  preceding  night  continued  until  12.10  a.  m.  Rain  began  again  at 
6.40  a.  m.  and  ended  at  7.35  a.  m.;  began  again  at  8.50  a.  m.  and  ended  at 
8.55  a.  m.  From  12.30  p.  m.  to  12.50  p.  m.  snow  was  mixed  with  rain.  The 
total  precipitation  for  the  day  was  0.02  inch. 

A  continuous  record  of  changes  in  the  meteorological  elements  at  Balti- 
more is  shown  in  the  accompanying  diagram.     (See  Fig.  121.) 


MARYLAND   WEATHER    SERVICE 


373 


The  storm  of  February  20-22,  1902. 

{Center  passes  east  of  Baltimore.) 

February,  1902,  was  remarkable  for  the  number  of  Gulf  storms 
experienced.  In  fact,  these  storms  were  a  conspicuous  feature  of  the 
entire  winter  of  1901-02.  While  the  great  majorit}'  of  our  storms 
follow  the  northern  route  across  the  Lake  region  in  a  normal  winter, 


Fig.  122.— The  Gulf  Storm  of  February  20,  1902. 


the  storms  of  February,  1902,  without  exception,  followed  the  southern 
path  and  crossed  the  horizon  of  Baltimore  with  remarkable  regularity 
by  way  of  the  Gulf  of  Mexico.  Occasionally  there  will  occur  a  series 
of  three  or  four  storms  in  regular  succession  following  this  track.  The 
area  of  cloudiness  and  rain  accompanying  a  Gulf  storm  passes  over  a 
given  locality  in  about  two  or  three  days;  this  is  followed  by  four  or  live 
days  of  fair  weather  before  the  approach  of  another  storm.  During 
the  winter  of  1901-02  there  was  a  remarkablv  regular  succession  of  these 


374 


THE    CLIMATE    OF   BALTIMORE 


Fig.  123.— The  Gulf  Storm  of  February  21,  1902. 


Fig.  124.— The  Gulf  Storm  of  February  22,  1902. 


25 


376 


THE    CLIMATE    OF   BALTIMORE 


storms,  the  period  of  rain  and  succeeding  fair  weather  covering  seven 
days  and  causing  the  unusually  long  continued  series  of  rainy  Sundays 
so  generally  commented  upon  at  the  time.  This  is  not  an  uncommon 
occurrence  but  the  regularity  of  the  succession  was  unusually  well 
marked.     (See  Figs.  126  and  127;  also  Fig.  113.) 

Why  storms  take  this  southern  course  with  such  unusual  frequency 
at  times  it  is  difficult  to  say.     Perhaps  all  that  can  be  said  in  explana- 


FiG.  126. — Normal  Paths  of  Storms  for  February  in  Black.     Average  Path  of 
Storms  for  February,  1902,  in  Red. 


tion  is  that  it  is  due  to  a  departure  from  the  normal  conditions  in  the 
general  circulation  of  the  atmosphere — some  unusual  movement  of  the 
large  persistent  areas  of  high  and  low  pressure  referred  to  in  an  earlier 
paragraph. 

One  of  the  most  notable  of  the  series  of  Gulf  storms  referred  to  above 
passed  over  Baltimore  on  February  21  and  22,  1902 — a  storm  which  will 
long  be  remembered  by  Baltimoreans  on  account  of  the  intensely  disagree- 


MARYLAND   WEATHER   SERVICE 


377 


able  combination  of  rain,  sleet,  snow,  and  high  winds  experienced.  The 
storm  originated  off  the  North  Pacific  coast  on  the  17th,  moved  rapidly 
southeastward,  reaching  the  Western  Gulf  coast  on  the  morning  of  the 
19th.  Here  it  lingered  for  a  day,  increasing  in  intensity  and  enlarging 
its  rain  area.  Moving  eastward  along  the  Gulf  coast  to  the  Atlantic, 
the  center  followed  the  coast  northward,  passing  to  the  east  of  Baltimore 
during  the  day  of  the  22d,  then  out  to  sea.  The  presence  of  an  area 
of  high  pressure  to  the  northeast  of  the  storm  assisted  in  producing  a 
steady  north  to  northeast  wind  during  the  21st.  The  official  records  of 
the  Weather  Bureau  describe  the  local  conditions  as  follows: 

February  20,  1902.     The  day  dawned  clear  and  cold.     A  thin  veil  of  cloud 
soon   appeared,   however,   increasing   in   thickness   as   the   day   advanced,   at 


SEPT.               OCT 

NOV 

DEC. 

JAN. 

FEB. 

MCH                   1 

7      14    21     28    5     12     19    26     2      9     16    23    30     7     14    21    28     4     H     18    25 

8     15   22 

8     15   22  1 

MOT. 
6    P 

1 

1 

1 

1 

1 
1 

6    A. 

1 

I 

1 

1 

j 
i 

1      ' 

■ 

..,'                                 .     .                                                                                                                                                  .                     1 

Fig.  127.— Diagram  of  Rainy  Sundays  of  the  Winter  of  1901-2. 


times  becoming  dark  and  threatening.  The  winds  throughout  the  day  were 
light  and  varying  in  direction  between  west  and  north,  and  changing  to 
south  in  the  evening.  The  temperature  rose  with  the  advance  of  the  day, 
but  barely  reached  the  melting  point  of  ice  even  at  mid-day.  Sleet  began  to 
fall  at  8  p.  m.,  turning  to  rain  during  the  night.  The  barometer  fell  slowly 
but  steadily  throughout  the  day  and  night. 

February  .11,  l'.i0.i.  The  rain  of  the  preceding  night  froze  as  it  fell,  cover- 
ing everything  with  a  thick  coating  of  ice.  On  trees,  telegraph  wires,  and 
all  exposed  objects,  the  ice  collected  to  a  thickness  of  an  inch  or  more,  the 
heavy  weight  causing  considerable  damage.  The  rain  continued  with  scarcely 
any  interruption  until  7.15  p.  m.;  at  times  it  fell  in  torrents.  Travel  upon 
the  streets  became  difficult  and  dangerous.  The  heavy  rains  of  the  afternoon 
converted  the  ice  upon  the  streets  into  a  heavy  slush.  The  temperature  re- 
mained nearly  constant,  varying  but  little  from  the  freezing  point  of  water. 
The  winds  were  northeast  to  north  all  day,  and  increasing  in  force,  not 
attaining    a    storm    velocity,    however.     The    barometer    continued    to    fall 


378  THE    CLIMATE    OF   BALTIMORE 

steadily  and  more  rapidly  toward  night.  The  total  precipitation  for  the  day 
was  2.13  inches.  Thq  center  of  the  depression  was  off  the  coast  of  North 
Carolina,  the  storm  moving  east  of  north  and  increasing  in  intensity. 

February  22,  1902.  A  rainy  day,  with  very  little  range  in  temperature, 
the  maximum  being  36°  and  the  minimum  34°.  The  barometer  reached  its 
lowest  reading  at  6  a.  m.,  rising  slowly  but  steadily  from  this  hour.  Fresh 
northerly  winds  prevailed.  The  heavy  rain  of  the  preceding  day  turned  to  a 
mist  at  11  a.  m.  and  was  accompanied  by  light  flurries  of  snow  between  3  p.  m. 
and  6  p.  m.  At  7.20  p.  m.  a  light  moist  snow  began  to  fall,  continuing  at 
8  p.  m.  The  total  precipitation  of  the  day  was  0.40  inch.  The  ice  remained 
upon  the  streets  most  of  the  day  in  spite  of  the  heavy  rains  and  was  a  source 
of  great  discomfort.  The  heavy  accumulation  of  ice  caused  much  damage 
to  trees  and  to  telegraph  and  electric  wires. 

February  23,  1002.  The  day  was  clear  and  somewhat  warmer  than  yester- 
day. The  snow  of  the  preceding  night  ended  about  midnight.  The  ice  on 
the  streets  rapidly  disappeared  with  the  increased  warmth.  The  day  was 
pleasant  and  the  atmosphere  balmy. 

Altogether  this  storm  was  one  of  the  most  disagreeable  experienced  in 

Baltimore.     (See  Figs.  122-125.) 

THE   BLIZZARD. 

When  storms  such  as  have  been  described  in  preceding  paragraphs 
are  accompanied  by  heavy  snow,  high  winds,  and  a  temperature  well 
below  the  freezing  point,  they  are  popularly  known  as  blizzards.  This 
type  of  storm  is  fortunately  of  infrequent  occurrence  in  the  Middle 
Atlantic  states.  When  they  have  occurred  it  has  been  in  connection 
with  a  Gulf  or  Southwest  storm.  An  invariable  accompaniment  of  the 
blizzard  is  the  presence  of  an  excessively  developed  area  of  high  baro- 
metric pressure  following  in  the  wake  of  the  depression,  causing  a  steep 
barometric  gradient  and  feeding  into  the  storm  center  with  great  energy 
the  cold  westerly  winds  of  the  anti-cyclone. 

Two  storms  of  this  type  are  especially  worthy  of  consideration  at 
some  length  owing  to  their  exceptional  severity  all  along  the  Atlantic 
coast — one  is  known  as  the  blizzard  of  March,  1888,  the  other  as  the 
blizzard  of  February,  1899.  The  former,  while  occurring  in  March, 
is  a  marked  instance  of  "  winter  lingering  in  the  lap  of  spring." 

The  Blizzard  of  March  11-13,  1888. 
The  daily  weather  charts  of  the  Weather  Bureau  for  March  11,  1888, 
show  the  existence,  in  the  morning,  of  an  area  of  high  pressure  (anti- 


MARYLAND   WEATHER   SERVICE 


379 


Fig.  128.— The  Blizzard  of  March  11,  1888. 


LOW 


^<i  \      HIGH 

I     /     \     " 


Fig.  120.— The  Blizzard  of  March  12,  1888. 


380 


THE    CLIMATE    OF   BALTIMORE 


cyclone)  centered  over  New  England,  and  another  of  unusual  extent  and 
energy  west  of  the  Mississippi  Eiver  with  its  center  over  the  Southern 
Eocky  Mountain  slope.  Between  these  two  anti-cyclones  there  was  a 
pronounced  trough  of  low  pressure  extending  from  Lake  Huron  to 
Florida.  Strong  east  to  southeast  winds  prevailed  in  the  Atlantic  coast 
states  with  heavy  rains,  excessive  in  the  South  Atlantic  states,  and  with 
temperatures  varying  from  30°  in  New  England  to  60°  in  South  Caro- 


Fig.  130.— The  Blizzard  of  March  13, 

Una.  To  the  westward  of  the  trough  of  low  pressure,  the  winds  were 
from  the  west  or  northwest,  with  snow  in  the  Lake  region  and  Ohio 
Valle}^,  and  rain  farther  south.  The  barometric  gradients  were  steep, 
and  the  temperatures  fell  rapidly  toward  the  northwest,  ranging  from 
50°  above  zero  in  Georgia  to  20°  below  zero  in  Northern  Minnesota. 
As  the  storm  moved  eastward  the  trough  of  low  pressure  changed  to  a 
well  developed  elliptical  depression,  with  its  center  off  the  coast  of  Hat- 
teras  by  10  p.  m.  of  the  11th;  at  the  same  time  the  storm  was  increasing 


MARYLAND  WEATHER   SERVICE 


381 


382  THE    CLIMATE    OF   BALTIMORE 

in  intensity,  causing  high  and  destructive  winds  along  the  Middle  Atlan- 
tic coast.  The  center  of  the  storm  moved  northward  near  the  coast, 
the  high  easterly  winds  of  the  11th  giving  way  to  high  off-shore  winds  by 
7  a.  m.  of  the  12th.  The  precipitation  continued  heavy  in  the  form  of 
snow  in  the  Middle  Atlantic  and  New  England  states.  The  cold  wave 
had  reached  the  Atlantic  coast  from  New  England  to  Virginia  by  the 
morning  of  the  13th.  The  center  of  the  storm  remained  off  the  coast  of 
Massachusetts  for  24  hours  and  then  moved  eastward  over  the  Atlantic. 
The  snowfall  over  the  southern  portion  of  the  New  England  states  was 
unprecedented  in  the  annals  of  that  section.  The  heavy  snows  and  high 
winds  attending  this  storm  caused  serious  interruption  to  telegraphic 
and  railway  communication  in  the  Middle  Atlantic  and  New  England 
states  from  the  11th  to  the  loth  of  the  month.     (See  Eigs.  128-131.) 

The  storm  passed  over  Baltimore  on  the  11th  and  the  early  morning 
of  the  12th.  The  following  paragraph  is  copied  from  the  local  official 
records : 

Light  rain  fell  at  intervals  until  noon  of  the  11th,  then  heavy  rain  until 
6.50  p.  m.,  when  it  changed  to  snow,  accompanied  by  high  northwest  winds. 
In  a  short  time  telegraphic  communication  was  cut  off  with  nearly  all  points. 
The  snow  storm  ended  during  the  night  of  the  llth-12th  and  was  followed 
by  cold  weather.  The  wind  continued  from  the  northwest  throughout  the 
12th,  attaining  a  maximum  velocity  of  40  miles  per  hour,  and  causing  the 
lowest  tide  in  many  years,  the  bottom  of  the  harbor  being  exposed  in  many 
places.  This  severe  storm  caused  an  almost  entire  suspension  of  business 
on  the  12th.  Reports  from  the  surrounding  country  and  from  the  Chesapeake 
Bay  show  the  storm  to  have  been  very  severe,  and  many  vessels  arriving  on 
the  14th  and  15th  reported  having  experienced  remarkably  rough  weather. 
The  tide  in  Baltimore  harbor  did  not  resume  its  normal  height  until  the  16th. 

The  Blizzard  of  February  12-14,  1899. 

This  storm  was  probably  the  most  remarkable  in  the  history  of  Balti- 
more. The  amount  of  snow  on  the  ground  at  the  close  of  the  storm 
was  the  greatest  noted  in  the  oflBcial  records  of  the  local  Weather  Bureau 
Ofl&ce  while  the  intense  cold  just  preceding  the  snow  storm  lowered  all 
existing  oflBcial  records.  The  winds  maintained  a  storm  velocity  for 
more  than  48  hours. 

The  cold  wave  which  preceded  the  blizzard  was  one  of  the  most  wide- 


MARYLAND   AYEATHER   SERVICE  383 

spread  as  well  as  one  of  the  most  severe  experienced  in  the  United  States, 
covering  the  entire  country  east  of  the  Eocky  Mountains  to  the  Atlantic 
coast,  and  from  the  Lake  region  to  the  Gulf  of  Mexico,  from  the  9th  to 
the  12th. 

The  distribution  of  atmospheric  pressure  over  the  northern  hemisphere 
during  this  period,  with  accompanying  weather  conditions,  was  of  peculiar 
interest  and  great  significance.  On  the  10th  of  February  practically 
the  entire  North  American  continent  was  covered  by  an  area  of  high 
barometric  pressure  (an  anti-cyclone)  with  a  pressure  of  over  31  inches 
at  the  center,  just  north  of  Montana,  a  pressure  only  occasionally  rec- 
orded. The  degree  of  cold  experienced  near  the  center  of  this  area  (65° 
below  zero)  was  exceeded  but  once  in  the  official  records  of  the  ^Yeather 
Bureau.  Upon  the  same  day  a  barometric  depression  (a  cyclone)  of  great 
intensity  covered  the  North  Atlantic  Ocean,  with  its  center  along  the 
same  parallel  of  latitude  (50°  north)  and  just  west  of  the  British 
Isles.  This  situation  gave  to  Southern  England  a  strong  southerly  wind 
which  raised  the  temperature  in  London  to  65°  above  zero,  a  degree  of 
heat  not  experienced  in  February  in  a  hundred  years  of  recorded  observa- 
tions. Thus  upon  the  same  day  and  along  the  same  parallel  of  latitude 
there  was  a  difference  of  130°  Fahrenheit.  The  contrast  was  even  more 
marked  in  the  United  States.  ^Yhile  the  minimum  of  65°  below  zero 
was  being  experienced  in  Western  Montana,  the  temperature  in  Western 
Washington  (just  across  the  Eocky  Mountains,  a  distance  of  less  than 
300  miles)  was  63°  above  zero,  a  difference  of  128°. 

This  anti-cyclone,  or  cold  wave,  which  overspread  the  United  States 
from  the  9th  to  the  11th  of  February,  caused  heavy  snows  to  the  south- 
east, along  the  line  of  advance.  By  the  morning  of  the  12th  a  baro- 
metric depression  began  to  develop  over  Northern  Florida,  and  heavy 
snow  was  falling  in  the  Gulf  states,  the  South  Atlantic,  ]\Iiddle  Atlantic, 
and  Southern  New  England  states,  with  fresh  to  brisk  north  to  northeast 
winds  and  falling  temperature.  At  the  same  time  the  anti-cyclone  in 
the  west,  maintaining  its  severity,  was  moving  southward  and  eastward. 
By  the  morning  of  the  13th  the  center  of  the  depression,  which  formed 
over  Florida  on  the  preceding  day,  moved  northward  along  the  coast, 


384 


THE    CLIMATE    OF   BALTIMORE 


s 


Fig.  132.— The  Blizzard  of  February  9,  1899. 


Fig.  133.— The  Blizzard  of  February  10,  1899. 


MARTLAXD   WEATHER   SERVICE 


385 


tow 


Fig.  134.— The  Blizzard  of  February  11,  1899. 


Fig.  135.— The  Blizzard  of  February  12,  1899. 


386 


THE    CLIMATE    OF   BALTIMORE 


Fig.  136.— The  Blizzard  of  February  13,  1899. 


Fig.  137.— The  Blizzard  of  February  14,  1899. 


MAKYLAXD   WEATHER    SERVICE 


387 


increasing  in  intensity  and  causing  high  northwest  winds  and  heavy 
snowfall.  The  center  of  the  storm  crossed  the  latitude  of  Baltimore 
during  the  day  of  the  13th  (Monday),  just  off  the  coast.  The  fall  of 
snow  during  this  day  was  the  heaviest  recorded  in  Baltimore  in  a  24 
hour  period.  The  temperature  during  the  entire  day  did  not  exceed 
10°  above  zero,  while  the  northwest  wind  blew  a  gale.  During  the  fol- 
lowing day  the  storm  continued  its  course  northeastward  along  the  coast 


Fig.  138. — Snow  on  the  Ground  after  the  Blizzard  of  February,  1899. 


and  out  over  the  Atlantic  Ocean   by  way  of  the  Grand  Banks  of  New- 
foundland.    (See  Figs.  132-138.) 

Tlie  local  conditions  of  the  weather  during  the  passage  of  the  cold 
wave  and  blizzard  described  above  are  indicated  in  the  following  extracts 
from  the  daily  journal  of  the  office  of  the  Weather  Bureau,  and  in  the 
accompanying  diagram  based  upon  the  records  of  the  self-registering 
instruments : 

February  9.  ]89!>.  The  day  was  rloar  and  much  colder  than  that  of  the 
8th.     The  maximum   temperature  of  the  day  was   the   lowest  maximum   re- 


388  THE    CLIMATE    OF   BALTIMORE 

corded  in  Baltimore,  namely  7°.  There  was  a  light  fog  in  the  morning.  The 
winds  were  brisk  to  high,  reaching  a  maximum  velocity  of  25  miles  per 
hour.  At  8  p.  m.  snow  covered  the  ground  to  a  depth  of  11.3  inches.  The 
ice  in  the  harbor  has  increased  in  thickness  to  two  inches. 

February  10,  1899.  A  clear  day.  Severe,  cold  weather.  The  maximum 
temperature  was  3°,  the  minimum  7°  below  zero,  the  mean  2°  below  zero, 
the  lowest  in  the  official  records  for  the  maximum,  minimum,  and  mean  for 
a  day.  Much  suffering  resulted  from  the  intense  cold.  Several  mortormen 
were  overcome,  and  were  revived  with  difficulty.  A  number  of  persons  were 
picked  up  out  of  the  snow  drifts  benumbed  and  unconscious.  The  suffering 
among  the  poor  was  very  great.  A  series  of  accidents  followed  the  sudden 
thawing  of  water  in  the  water  pipes  when  fires  were  started  in  the  morning. 
Ten  inches  of  snow  covered  the  ground  at  8  p.  m.,  and  the  ice  in  the  harbor 
increased  to  six  and  eight  inches  in  thickness.  The  two  ice  boats  were  busy 
all  day  in  their  attempts  to  keep  the  channel  clear,  but  the  ice  formed  almost 
as  fast  as  it  was  broken. 

February  11,  1899.  A  clear  day.  The  minimum  temperature  was  6°  be- 
low zero.  A  light  fog  prevailed  in  the  morning.  Light  snow  began  to  fall 
at  5.35  p.  m.  and  continued  into  the  night.  The  snow  on  the  ground  at  8 
p.  m.  was  9.7  inches  in  depth.  There  continues  to  be  much  suffering  from 
cold,  and  one  death  from  exposure  is  reported. 

February  12,  1899.  A  cloudy  day,  with  slowly  rising  temperature.  North- 
east storm  warnings  were  ordered  up  in  the  forenoon.  The  following  tele- 
gram was  received  from  the  Central  Office  in  Washington:  "Heavy  snow  is 
indicated  this  afternoon  and  to-night.  Notify  railroads  and  transportation 
companies."  The  snow  which  began  yesterday  at  5.35  p.  m.  became  heavy  at 
8.15  p.  m.,  then  changed  again  to  light  snow  during  the  night.  It  continued 
throughout  the  day.  About  five  inches  of  snow  fell  during  the  day;  the 
depth  of  snow  on  the  ground  at  8  p.  m.  was  14.5  inches.  The  weight  of  the 
snow  had  crushed  a  number  of  small  sheds  and  a  few  wooden  structures. 
To-day  the  President  Street  freight  sheds  gave  way,  owing  to  the  accumula- 
tion of  snow  on  the  roof,  and  about  300  feet  of  the  building  fell;  the  damage 
amounted  to  about  $20,000.  The  ice  in  the  harbor  is  6  to  8  inches  thick. 
Navigation  is  practically  suspended.  Only  heavy  steel  steamships  are  able 
to  move.  Trains  are  late  and  irregular.  Much  suffering  continues  among 
the  poor. 

February  13,  1899.  A  cloudy  day.  Heavy  snow  fell  all  day;  the  24-hour 
fall  was  15.5  inches.  The  depth  of  snow  on  the  ground  at  8  p.  m.  was  30 
inches,  the  greatest  recorded  in  the  official  records.  Brisk  to  high  north  to 
northwest  winds  attained  a  maximum  velocity  of  28  miles  per  hour.  The 
continued  high  blustery  winds  and  the  increasing  snowfall  combined  to  pro- 
duce a  typical  blizzard.  Railroad  traffic  was  interrupted  at  an  early  hour. 
The  street  railways  struggled  to  continue  service,  but  the  lines  closed  one 
by  one,  and  none  were  in  operation  by  nightfall.  Much  suffering  continues. 
At  least  a  score  of  people  were  overcome  by  the  cold  during  the  day.  Birds 
are  reported  perishing  in  large  numbers  from  cold  and  lack  of  food. 


VOLUME  2,  PLATE  XX. 


NOON 


MD" 


NOON 


MDT. 


MARVLANO  WEATHER  SERVICE. 


VOLUME  2,  PLATE  ) 


/-^\-^ ./- 


I  l\\\l\\\l-\ ^W 


"      ■*      «      5      «      "      "    '2     1°     5      7      6      6      5      9      s      10     10     s      10     e      1      3      5      3      4      t      ■•      4      2       5      5      3      6      6      5      7      7      II      10     II      (I      l(      II      2      10     12     1J     13     15     17     21    20     /a     re     1(      ((     (0     II    '6 


,8     13      979*«86222 


lloimLY  Observations  at  BAr/rrMORE  Purinq  the  Buzzard  and  Cold  Wave  op  Febboary  9-14,  1899. 


MARYLAND   WEATHER   SERVICE  389 

February  14,  1899.  A  cold  day  with  bright  sunshine.  Snow  ended  at  11.10 
p.  m.  yesterday;  one  inch  of  snow  was  recorded  this  morning.  Total  snow 
depth  at  8  p.  m.,  28  inches.  The  ice  in  the  harbor  is  10  inches  thick.  The 
city  is  practically  snowbound.  There  was  no  mail  delivery,  no  railroad  move- 
ment, no  street  car  service.  Some  vessels  forced  their  way  out  of  the  harbor, 
but  they  were  few  in  number.  Much  work  is  being  done  by  the  city  and 
railroad  authorities  on  the  streets  and  lines  of  travel,  but  traflac  was  only 
partially  restored.  Much  suffering  continues.  One  man  was  found  frozen 
to  death  this  morning  within  six  doors  of  his  home.  A  milk  famine  is 
threatened.  There  has  been  a  genei'al  rise  in  the  price  of  commodities, 
especially  of  country  produce.  The  Merchants  and  Miners  Transportation 
Company  lost  the  steamship  Texas  this  morning.  The  vessel  was  run  ashore 
in  an  ineffectual  attempt  to  force  a  way  through  the  ice  and  sank. 

February  15,  1899.  A  clear  day.  Light  fog  in  the  morning.  Street  car 
service  was  resumed  to-day  in  part.  Trains  are  beginning  to  run  on  time. 
The  ice  in  the  harbor  is  one  foot  thick.  The  ice  boats  have  succeeded  in 
keeping  a  clear  channel  of  50  feet  width.  Four  arrivals  of  vessels  and  one 
departure  are  reported  for  to-day.     Snow  on  ground  at  8  p.  m.,  26  inches. 

Areas  of  Fair  Weather  (Axti-cycloxes), 

In  the  preceding  pages  the  cyclonic  type — or  unsettled  weather — has 
been  described  in  considerable  detail.  The  characteristic  conditions  of 
this  type  are  cloudiness,  rainfall  or  snowfall,  brisk  to  high  winds,  a 
relatively  high  temperature,  and  a  low  barometric  pressure,  with  winds 
converging  toward  the  central  area  of  low  pressure. 

In  the  Middle  Atlantic  states  the  cyclonic  type  dominates  the  weather 
conditions  somewhat  less  than  half  the  time,  basing  the  calculation  upon 
the  number  of  days  in  the  year  during  which  some  rain  or  snow  falls. 
The  annual  number  of  days  with  precipitation  at  Baltimore  has  varied 
from  114  to  224  with  a  mean  of  170.  This  implies  that  during  some- 
what over  half  the  year  the  anti-cyclonic,  or  fair  weather  type,  prevails. 
The  chief  characteristics  of  anti-cyclonic  areas  are :  Barometric  pres- 
sure higher  than  that  over  surrounding  areas;  a  system  of  comparatively 
light  winds,  diverging  from  the  central  portion  of  the  area :  comparatively 
clear  skies;  and  relatively  low  temperature.     (See  Figs.  85  to  89.) 

High  areas,  or  anti-cyclones,  have  already  been  described  incidentally  in 
the  preceding  discussion  of  storms.  They  are  most  numerous  and  more 
intensely  developed  during  the  winter  season,  when  they  move  in  rapid 
succession  from  the  central  continental  areas,  in  the  extreme  Northwest, 


390  THE    CLIMATE    OF   BALTIMORE 

along  the  eastern  slope  of  the  Kocky  Mountains,  eastward  or  southeast- 
ward across  the  United  States. 

When  these  areas  grow  to  unusual  proportions  and  develop  a  baro- 
metric pressure  greatly  in  excess  of  the  pressure  in  areas  along  their  line 
of  eastward  progress,  they  constitute  our  "  cold  waves."  There  is  no 
sharp  line  of  separation,  however,  between  the  cold  w-ave  and  the  winter 
anti-cyclone — no  more  than  there  is  between  the  storm  and  the  barometric 
depression  technically  known  as  the  cyclone.  The  anti-cyclone  attains 
its  greatest  severity  when  a  barometric  depression  develops  in  advance 
of  it,  causing  an  energetic  inflow  of  cold  northwest  winds  into  the  western 
portion  of  the  depression.  In  area  and  in  rate  of  movement  the  cyclone 
and  anti-C3'clone  resemble  one  another ;  in  the  character  of  attendant 
weather  conditions  they  are  in  most  respects  the  exact  opposite. 

The  difference  in  the  character  of  the  weather  prevailing  over  cyclonic 
and  anti-cyclonic  areas  is  strikingly  exhibited  in  the  weather  chart  for 
the  2d  of  February,  1902,  reproduced  in  Fig.  115,  showing  the  actual 
condition  of  the  weather  at  8  a.  m.  as  reported  by  the  observers  of  the 
United  States  Weather  Bureau  in  all  parts  of  the  United  States  and 
Southern  Canada.     (See  also  Fig.  89,  page  325.) 

A  storm,  or  cyclone,  of  great  extent  and  energy  prevailed  over  the 
eastern  portion  of  the  country,  with  its  central  area  of  low  barometric 
pressure  over  Pennsylvania  and  Maryland.  The  area  of  clouds  extended 
from  the  Atlantic  Ocean  westward  to  the  Mississippi  Valley,  and  from 
the  Great  Lakes  southward  to  the  Gulf  coast.  The  region  over  which 
rain  or  snow  was  falling  at  the  time  of  observation,  8  a.  m.,  w^hile  more 
limited  in  extent  still  covered  a  considerable  area,  comprising  practically 
all  of  the  Kew  England  and  Middle  Atlantic  states,  Ohio,  Kentucky,  and 
the  eastern  half  of  the  Lake  region.  In  the  Eastern  Gulf  states  and  the 
Atlantic  coast  states  as  far  north  as  Virginia  the  rains  of  the  preceding 
24  hours  were  very  heavy,  in  some  localities  exceeding  an  inch  and  a  half. 
It  will  be  observed  also  that  the  winds  within  the  area  just  outlined 
blew  in  the  main  toward  the  central  area  of  low  pressure,  and  that 
the  temperatures  were  markedly  higher  within  the  cyclonic  area  than 
in    the    anti-cyclonic    area    immediately    to    westward    of    the    storm 


MARYLAND   WEATHER   SERVICE  391 

area.  The  isotherms,  or  lines  of  equal  temperature,  bent  far  northward 
in  advance  of  the  storm  center,  where  easterly  to  southerly  winds  pre- 
vailed ;  to  the  west  of  the  center  the  cold  northwest  winds  reached  far 
to  the  south. 

High  atmospheric  pressure  prevailed  over  the  area  west  of  the  Missis- 
sippi River,  the  highest  barometer  being  over  Kansas  and  Oklahoma. 
The  skies  were  mostly  free  from  clouds,  the  winds  blew,  in  general,  away 
from  the  central  region  of  high  pressure,  the  temperatures  were  compara- 
tively low,  while  the  isotherms  were  bent  southward,  with  a  maximum 
dip  near  the  center  of  the  area. 

The  intimate  relationship  existing  between  wind  direction  and  the 
trend  of  the  isotherms,  as  explained  in  preceding  pages,  is  strikingly 
exhibited  in  this  chart.  The  difference  in  temperature  between  the 
centers  of  cj'clone  and  anti-cyclone  along  the  40th  parallel  of  latitude 
at  8  a.  m.  was  fully  50°. 

The  successive  changes  in  weather  conditions  at  Baltimore  as  these 
two  systems — the  cyclone  and  anti-cyclone — moved  eastward  are  shown 
in  the  accompanying  diagram.     (See  Fig.  117.) 

The  weather  conditions  over  the  United  States  do  not  always  exhibit 
these  cyclonic  and  anti-cyclonic  systems  so  well  defined,  but  during  the 
winter  season  their  outlines  are  nearly  always  easily  recognizable.  In 
place  of  a  definite  succession  of  "  highs  "  and  "  lows  "  such  as  are  shown 
by  this  chart  of  February  2,  1902,  there  may  be  a  number  of  ill-defined 
and  scattered  centers  of  high  and  low  pressure,  causing  a  period  of 
unsettled  weather  conditions, 

COLD   WAVES. 

When  anti-cyclonic  waves  are  accompanied  by  a  fall  of  20°  or  more 
(exclusive  of  the  diurnal  fluctuation)  to  a  stated  minimum,  within  a 
period  of  24  hours,  they  are  technically  known  as  cold  waves.  Sudden 
changes  in  temperature  such  as  are  here  described  are  of  comparatively 
frequent  occurrence  in  the  northern  tier  of  states,  but  do  not  reach  as  far 
south  as  Baltimore  in  any  great  numbers.  In  their  progress  eastward 
and  southward  these  cold  waves  lose  much  of  the  severity  shown  when 

20 


393  THE    CLIMATE    OF   BALTIMORE 

they  first  enter  this  country  from  the  Canadian  Northwest  Territory. 
By  the  time  they  reach  the  Atlantic  coast  many  of  them  have  lost  the  dis- 
tinguishing marks  of  a  genuine  cold  wave.  In  the  official  records  of  the 
United  States  Weather  Bureau  we  find  that  out  of  a  dozen  or  more  anti- 
cyclones which  enter  this  country  every  winter  in  the  extreme  Northwest 
as  cold  waves,  but  three,  on  an  average,  retain  sufficient  of  their  severity  to 
be  classed  as  cold  waves  as  they  pass  over  Baltimore. 

In  some  winters  Baltimore  has  been  entirely  free  from  them.  This 
was  the  case  in  the  winters  of  1873-4,  1885-6,  and  1889-90.  Some  times 
as  many  as  six  have  been  experienced  in  one  season,  as  in  the  winters  of 
1881-2,  1884-5,  and  1903-4.  From  1870  to  1904  the  monthly  distribu- 
tion of  cold  waves  *  in  Baltimore  has  been  as  follows : 


November. 

December. 

January. 

February. 

March. 

Total. 

8 

19 

23 

25 

17 

93 

The  Cold  Wave  of  December  13-15,  1901. 

The  eastward  and  southward  progress  of  cold  waves  from  the  north- 
west is  well  exemplified  in  the  weather  charts  of  the  United  States 
Weather  Bureau  for  the  13th,  14th,  and  15th  of  December,  1901. 

At  8  a.  m.  of  the  13th  there  were  two  well  defined  and  extensive  areas 
of  high  pressure,  or  anti-cyclones,  shown  upon  the  chart:  One  covered 
the  eastern  section  of  the  country,  comprising  all  of  the  Atlantic  coast 
states,  with  the  maximum  barometer  over  Nova  Scotia;  the  other  spread 
over  most  of  the  country  west  of  the  Mississippi  Eiver,  with  the  center 
to  the  north  of  Montana.  Between  these  two  vast  anti-cyclones,  there 
was  a  narrow  trough  of  relatively  low  pressure,  extending  from  the  Upper 
Lake  region  to  the  Gulf  of  Mexico.  In  advance  of  this  trough  of  low 
pressure,  or  elongated  cyclonic  depression,  the  temperatures  rose  rapidly 
under  the  influence  of  strong  southerly  winds.  To  westward  of  the 
trough  the  cold  northwest  winds  blowing  out  of  the  well  developed  anti- 
cyclone brought  freezing  weather  far  down  into  the  southern  states.  The 
western  anti-cyclone  moved  southeastward  as  a  great  wave  of  cold  air, 
closely  following  the  cyclonic  depression,  causing  a  very  steep  gradient 

^  See  page  128  for  details  of  cold  waves. 


MARYLAND   WEATHER    SERVICE 


393 


Fig.  139.— Cold  "Wave  of  December  13,  1901. 


/  HIGH 


Fig.  140.— Cold  Wave  of  December  14,  1901. 


394 


THE    CLIMATE    OF   BALTIMORE 


in  temperature  from  west  to  east  along  the  wave  front.  In  a  straight 
line  from  Central  Alabama  northwestward  to  the  Dakotas,  from  the 
center  of  the  cyclone  to  the  center  of  the  anti-cyclone,  there  was  a  fall 
of  100°  at  8  a.  m.  of  the  14th;  Montgomery,  Ala.,  reported  a  temperature 
of  70°  above  zero,  and  Bismarck,  N.  Dak.,  a  temperature,  at  the  same 
hour,  of  30°  below  zero.  By  8  a.  m.  of  the  following  day,  the  15th,  the 
isotherm  of  20°  extended  along  the  West  Gulf  coast.    The  cold  wave  did 


Fig.  141.— Cold  Wave  of  December  15,  1901. 


not  reach  Baltimore  until  the  forenoon  of  the  15th,  and  the  Atlantic 
coast  on  the  morning  of  the  16th. 

When  the  cold  wave  takes  a  southeastward  path,  as  it  did  in  this  in- 
stance, the  freezing  temperatures  very  frequently  reach  the  Gulf  coast 
states  from  12  to  24  hours  in  advance  of  their  occurrence  in  the  Middle 
Atlantic  and  ISTew  England  states.     (See  Figs.  139-141.) 


MARYLAND   WEATHER   SERVICE  395 

The  Cold  Wave  of  Fehruanj  10-13,  1S99. 

This,  the  most  intense  and  wide-spread  anti-c^'clone  experienced  in 
many  years  in  this  country,  has  already  been  referred  to  in  preceding 
pages  in  the  discussion  of  the  ''  Blizzard  of  Februar}^,  1899  "  with  which 
it  was  associated.     (See  Figs.  132  to  138,  and  PL  XX.) 

Eecords  of  minimum  temperatures  long  undisturbed  were  lowered  in 
many  states  east  of  the  Eocky  Mountains  during  the  eastward  and  south- 
ward progress  of  this  cold  wave.  It  extended  even  to  the  West  India 
Islands.  At  Havana,  Cuba,  the  temperature  fell  to  54°  on  the  13th.  At 
Washington,  D.  C,  a  minimum  of  15°  below  zero  was  reported  on  the 
11th.  The  lowest  temperature  in  the  official  record  at  Baltimore  was  7° 
below  zero,  but  temperatures  considerably  lower  were  reported  from  the 
suburban  districts.  In  passing  across  the  Gulf  states  zero  temperatures 
were  experienced  on  the  13th  as  far  south  as  the  coast. 

Cold  waves  probably  differ  from  anti-cyclones  in  general  only  in  the 
intensity  of  their  development  and  in  the  circumstance  of  being  preceded 
by  a  cyclonic  depression.  The  conditions  most  favorable  for  the  develop- 
ment of  anti-cyclones  are  the  clear  skies  and  dry  quiet  atmosphere  of  the 
extreme  Xorthwest — conditions  which  favor  rapid  terrestial  radiation. 
The  general  eastward  drift  of  the  air  in  latitudes  between  30°  and  60° 
north  latitude  carries  the  anti-cyclones,  when  once  formed,  along  in  the 
general  current.  With  the  eastward  movement  there  is  a  southward  ten- 
dency of  these  anti-cyclones,  due  probably  to  the  centrifugal  force  of  the 
earth's  axial  revolution. 

These  anti-cyclones,  like  the  cyclones,  move  across  the  continent  at 
irregular  intervals,  occupying  four  to  five  days  in  travelling  from  the 
Pacific  to  tlie  Atlantic  coast,  according  to  their  extent  and  path.  In 
most  cases  the  center  of  the  anti-cyclone,  or  high  area,  passes  eastward 
to  the  north  of  Baltimore,  but  frequently  the  center  passes  directly  over 
Maryland,  and  occasionally  to  the  south.  Those  passing  along  the 
northern  route  are  most  likely  to  bring  very  low  temperatures  to  our  lati- 
tudes, but  this  is  not  always  true.  In  fact,  in  the  case  of  the  cold  wave 
of  December  13th  to  15th,  1901,  described  above,  the  center  passed 
directly  over  Baltimore.    At  such  times  the  temperatures  duo  in  the  cold 


396  THE    CLIMATE    OF   BALTIMORE 

winds  from  the  north  or  northwest,  which  accompany  all  cold  waves, 
are  further  lowered  by  the  rapid  terrestial  radiation  which  takes  place 
in  the  calm  clear  centers  of  the  anti-cycloues,  especially  during  the  night 
hours. 

The  low  temperatures  associated  with  cold  waves  do  not  as  a  rule  con- 
tinue more  than  three  or  four  days.  By  the  fourth  or  fifth  day  the  normal 
minimum  for  the  month  is  again  reached.  (See  pages  117  to  133  for 
additional  details  of  cold  days.) 

The  Origin  of  Cold  Waves. 

The  cold  waves  of  the  United  States  have  been  explained  in  various 
ways  by  those  who  have  studied  their  history.  One  hypothesis  accounts 
for  them  by  attributing  them  to  the  upper  westerly  winds  which  are 
drawn  across  the  Eocky  Mountain  range  into  the  cyclonic  depressions  to 
the  east  of  the  mountains.  The  clear  dry  air  which  descends  along  the 
eastern  slope  cools  by  radiation  into  space  during  the  long  winter  nights 
to  such  an  extent  as  to  greatly  overbalance  the  warming  effect  due  to 
compression  and  insolation  as  the  air  descends  from  the  higher  to  the 
lower  levels. 

Another  theory  attril)utes  them  to  the  southward  movement  of  detached 
masses  of  the  dry  cold  atmosphere  which  form  rapidly  during  tJie  winter 
months  in  the  region  to  the  west  of  Hudson's  Bay. 

A  third  hypothesis  refers  them  to  slowly  descending  currents  of  the 
anti-trades  which  flow  from  the  equator  toward  the  Arctic  region  in 
the  higher  levels  of  the  atmosphere,  the  loss  of  heat  during  the  long 
journey  over  the  continent  being  sufficient,  by  the  time  they  reach  the 
surface  in  latitudes  between  60°  and  70°,  to  account  for  the  low 
temperatures  observed  in  our  cold  waves. 

These  explanations  appear  plausible,  and  it  may  be  that  the  excessively 
low  temperatures  observed  in  our  severest  cold  waves — temperatures 
of  50°  to  60°  below  zero — must  be  attributed  to  a  combination  of  two 
or  all  of  the  causes  mentioned. 

Eecent  investigations  into  the  conditions  at  very  great  elevations  over 
the  equator  have  shown  the  existence  of  low  temperatures  never  experi- 


MARYLAND    WEATHER    SERVICE  397 

enced  at  the  earth's  surface,  e\en  within  the  Arctic  Circle.  In  the  sum- 
mer of  1906  M.  Teisserenc  de  Bort  and  Mr.  Rotch  by  means  of  pilot  bal- 
loons succeeded  in  obtaining  records  of  a  temperature  of  123°  below  zero 
from  an  elevation  of  about  nine  miles  above  the  earth  within  the  Tropics. 

THE  COLD  VS^INTER  OF  1903-4. 

One  of  the  coldest  winters  on  record  in  the  history  of  Baltimore  was 
that  of  1903-4.  In  some  respects  it  was  more  severe  than  the  winter  of 
1855-6,  generally  regarded  as  the  hardest  winter  experienced  in  the 
Middle  Atlantic  states.  There  were  very  few  excessively  cold  days,  the 
low  average  temperature  for  the  three  winter  months  being  due  to  long 
continued  moderate  cold.  The  average  temperature  for  the  entire  season 
was  6.3°  per  day  below  the  normal.  The  winter  most  nearly  approach- 
ing this  in  continued  cold  was  that  of  1892-3,  with  a  daily  departure  of 
5.2°  below  the  normal  for  the  season.  Next  in  order  comes  the  famous 
winter  of  1855-6  with  an  average  daily  departure  of  4.6°  below  the  sea- 
sonal average. 

With  a  minimum  of  2°  above  zero  on  January  5,  the  winter  was  not 
at  all  remarkable  for  low  temperatures.  The  trying  combination  of 
intense  cold,  high  wind,  and  snow,  occasionally  experienced  in  Baltimore 
winters,  was  entirely  lacking.  The  season  was  characterized  by  an 
almost  unbroken  period  of  moderately  cold  weather,  and  an  unusual 
frequency  of  snowfall,  rather  than  an  excessive  quantity.  The  ice  was 
very  heavy;  an  abundant  crop  was  cut  before  the  first  of  January,  an 
unusual  event  for  the  vicinity  of  Baltimore.  The  ice  in  the  Bay 
and  harbor  impeded  navigation  to  a  greater  extent  than  for  many  years. 
For  a  time  during  the  second  decade  of  January,  and  again  in  the 
middle  of  February,  ice  covered  the  entire  Bay  from  the  Susquehanna  to 
the  Patuxent,  and  even  the  larger  steamers  were  obliged  to  remain  in 
port. 

There  was  but  a  single  "  cold  wave "  in  the  technical  sense  of  the 
term,  that  is,  a  fall  of  20°  within  24  hours  to  a  minimum  of  20°,  neglect- 
ing the  usual  diurnal  variation  in  temperature ;  this  occurred  on  the 
26th  of  December  with  a  fall  of  22°  and  a  minimum  of  11°. 


398  THE    CLIMATE    OF   BALTIMORE 

There  was  less  than  the  average  amount  of  precipitation  for  the  winter 
season,  including  snow  and  rain.  The  deficiency  for  the  three  montlis 
aggregated  over  four  inches.  The  frequency  of  days  witli  snow  was 
particularly  significant.  The  normal  number  of  sno^vs  in  the  winter 
season  at  Baltimore  is  12;  in  1903-4  there  were  22.  It  is  a  well  recog- 
nized fact  that  a  snow  cover  lowers  the  temperature  materially.  Hourly 
observations  of  temperature  at  Baltimore  during  a  period  of  ten  years 
show  that  on  days  when  the  ground  was  covered  with  snow  the  mean 
temperature  was  10°  lower  than  on  the  normal  winter  day.  The  tem- 
perature is  lowered  during  the  night  by  reason  of  the  intense  radiation 
from  the  snow  surface;  during  the  day  much  of  the  heat  which  would 
otherwise  go  to  W' arm  the  atmosphere  is  employed  in  melting  and  vaporiz- 
ing the  snow. 

Some  of  the  most  significant  departures  from  normal  winter  condi- 
tions are  indicated  in  the  following  comparison : 

THE    WINTERS    OF    1903-4    AND    1889-90    CONTRASTED. 

TO  ,         Cold  Warm 

w-^Tf        Winter     Winter 
W  inter.       i9n3_4.      1889-90. 

Number  of  days  with  a  mean  temperature  below  32°. .     34  66  11 

Number  of  days  with  a  mean  temperature  above  40°.  .20  8  63 

Number  of  days  with  a  min.  temperature  below  32°.  .     59  78  28 

Number  of  days  with  a  max.  temperature  below  32°.  .14  21  5 

Number  of  days  with  a  temperature  of  20°  or  less. ...     18  39  6 

Number  of  days  with  a  measurable  amount  of  snow.  .12  22  7 

Total  depth  of  snowfall  in  inches 24  26  5 

Total  precipitation  (rain  and  melted  snow)   in  inches.     10  6  7 

Number  of  days  with  snow  on  ground 22  40  — ' 

First  killing  frost  occurred Nov.  7  Nov.  7  Nov.  6 

Last  killing  frost  occurred Apr.  4  Apr.  17  Apr.  2 

^  No   record. 

The  following  notes  on  ice  conditions  during  the  winter  of  1903-4  are 
taken  from  the  daily  journal  of  the  United  States  Weather  Bureau : 

1903. 
Nov.     7.     First  ice. 
Dec.    16.     Ice  4  to  5  inches  thick. 

1904. 
Jan.      4.     Ice    6    to   7    inches   on    Druid   Hill    Lake.     Bay   frozen    over   from 
Sharp's  Island  to  Ft.  Carroll.     Ice  off  Sharp's  Island  8  inches 
thick. 


MAKYLAXD   WEATHER    SERVICE  399 


Ice  on  Druid  Hill  Lake,  S  inches. 

Heavy  ice  50  to  60  miles  down  the  Bay.     All  sailing  vessels  and 

small  steamers  tied  up. 
Heavy   drift   ice    in   Bay — flows    5    feet    thick,    making   navigation 

dangerous. 
From  Sandy  Point  up  the  harbor  ice  jams  delay  strongest  steamers. 
Ice  on  Druid  Hill  Lake,  12  inches. 
Ice  fields  from  Susquehanna  to  Patuxent.     Passable  only  by  means 

of  ice  boats. 
Whole  Bay  solidly  frozen  over  down  to  Cove  Point.     Ice  3  to  18 

inches.     Navigation  suspended. 
Fields  of  heavy   drift  ice  reported   as   far  south  as   the  Potomac. 

Conditions  the  worst  of  the  winter. 
Some    ice    floes    have   an    area    of   400    to    500    acres.     Navigation 

hazardous. 
Navigation  practically  suspended. 
Ice  in  harbor  again  becoming  a  serious  obstacle  to  navigation  in 

spite  of  the  good  work  of  the  ice  boats. 
Ice  conditions  nearly  as  bad  as  at  any  time  of  the  season. 
The  steamer  Alabama  from  Old  Point  ran  into  drift  ice  extending 

from  shore  to  shore  only  30  miles  above  Old  Point. 
The  largest  steamers  are  remaining  in  port  on  account  of  ice. 
The  ice  in  the  Bay  is  causing  no  more  serious  trouble. 

THE   WARM   WINTER   OF    1889-90, 

The  winter  of  1889-90  was  quite  as  remarkable  for  its  mildness  as  that 
of  1903-4  was  for  its  severity.  The  excess  in  temperature  was  even 
greater  than  the  deficiency  of  the  winter  in  1903-1,  being  nearly  8°  above 
normal  per  day,  as  compared  with  6°  below  in  1903-1.  The  winter 
passed  without  a  single  cold  wave.  The  seasonal  snowfall  (5  inches) 
was  the  lightest  since  1871,  or  since  the  establishment  of  the  local  office 
of  the  Weather  Bureau.  The  number  of  days  upon  which  snow  fell  was 
but  7  as  compared  with  an  average  number  of  12  and  as  compared  with 
22  in  1903-4.  There  were  but  six  days  of  the  season  on  which  the 
temperature  fell  as  low  as  20° ;  in  1903-4  there  were  39  days  with  a 
temperature  of  20°  or  below. 

On  12  days  of  the  winter  of  1889-90  the  tempernture  rose  to  points 
never  reached  upon  those  days  in  30  years,  and  upon  two  days  the  highest 
temperature  recorded  in  December  and  January  occurred,  namely,  73° 
on  December  26,  1889  and  January  13,  1890. 


1904. 

Jan. 

6. 

Jan. 

8. 

Jan. 

12. 

Jan. 

13. 

Jan. 

18. 

Jan. 

19. 

Jan. 

20. 

Jan. 

29. 

Jan. 

31. 

Feb. 

2. 

Feb. 

15. 

Feb. 

18. 

Feb. 

19. 

Feb. 

21. 

Mar. 

2. 

400  THE    CLIMATE    OF   BALTIMORE 

THE    DISTRIBUTION    OF    ATMOSPHERIC    PRESSURE    DURING    THE    COLD 
WINTER    OF    1903-4    AND    THE    WARM    WINTER    OF 

1889-90.  (PI.  XXIY.) 
The  character  of  the  weather  ol  a  given  locality  depends,  so  far  as 
temperature  and  winds  are  concerned,  upon  the  general  distribution  of 
the  atmospheric  pressure  over  a  large  area  surrounding  the  locality. 
The  pressure  determines  the  wind  direction  directly,  and  the  winds,  in 
turn,  modify  the  temperature.  If  we  examine  carefully  a  chart  showing  the 
normal  winter  atmospheric  pressure  over  the  North  American  continent, 
we  find  that  the  barometer  is  relatively  low  over  Labrador  and  surround- 
ing regions,  and  high  over  the  interior  of  the  continent  and  over  the 
central  and  southern  states.  Such  a  distribution  of  pressure,  as  shown 
in  the  discussion  of  cyclones  and  anti-cyclones  in  preceding  pages,  gives 
to  Baltimore  and  vicinity  a  prevailing  west  to  northwest  wind.  If  this 
normal  winter  distribution  of  pressure  is  disturbed,  a  change  in  the 
prevailing  wind  directions  will  result,  with  attendant  changes  in  tempera- 
ture, and  other  factors.  If  we  examine  the  distribution  of  the  mean 
seasonal  pressure  over  the  country  during  the  winter  of  1903-i,  we  find 
a  distribution  differing  greatly  from  the  normal.  The  centers  of  the 
areas  of  high  pressure  during  December,  January,  and  February  were  to 
the  west  and  northwest  of,  or  over  Baltimore,  but  the  areas  were  much 
more  extensive  and  intensely  developed,  causing  a  steady  and  strong 
flow  of  cold  northwest  winds  during  the  entire  season.  A  winter  season 
may  be  severe  by  reason  of  a  considerable  percentage  of  exceptionally 
cold  days,  or  by  reason  of  long  continued  moderate  cold.  The  season  of 
1903-4  belonged  to  the  latter  class.  If  we  examine,  on  the  other  hand, 
the  distribution  of  seasonal  pressure  during  the  winter  of  1889-90,  we 
find  a  totally  different  condition — a  wide  departure  from  the  normal 
type.  The  barometer  was  persistently  high  over  the  southeastern  por- 
tion of  the  country — an  extension  westward  of  the  permanent  area  of  high 
pressure  over  the  Atlantic  Ocean.  This  distribution  of  pressure  gave 
to  Baltimore  and  vicinity  a  prevailing  wind  direction  from  the  south 
and  southwest  during  December  and  January,  and  an  easterly  direction 
during  February.  As  opposed  to  the  prevailing  northwest  winds  of  an 
average  winter,  these  directions  all  give  a  relatively  high  temperature. 


MAEYLAND   WEATHER   SERVICE  401 

In  addition,  the  storm  tracks  of  the  winter  1889-90  were,  without  excep- 
tion, far  to  the  north  of  Baltimore,  diifering  widely  from  the  usual  dis- 
tribution, and  causing  an  unusual  percentage  of  southerly  winds. 

The  influence  of  the  mean  distribution  of  atmospheric  pressure  upon 
the  general  character  of  the  weather  will  be  further  considered  in  con- 
nection with  the  discussion  of  weather  conditions  in  other  seasons. 

THE  VARIABILITY  OF  WINTER  WEATHER. 

In  the  middle  latitudes  winter  is  a  season  of  great  contrasts.  This  is 
particularly  characteristic  of  regions  lying  near  the  paths  of  the  storm 
centers.  It  is  not  difficult  to  find  a  reason  for  the  great  and  sudden 
fluctuations  experienced  when  we  bear  in  mind  the  conditions  described 
in  the  preceding  pages.  In  our  latitudes  there  is  a  continuous  succes- 
sion of  atmospheric  waves  moving  from  west  to  east  across  the  continent. 
Within  the  troughs  of  these  waves  as  they  move  eastward  we  find,  in  any 
well  developed  type,  warm  southerly  winds  which  raise  the  temperature 
of  the  localities  over  which  they  blow  approximately  10°  above  the 
normal  for  the  time  and  place.  As  the  crest  passes  over  the  locality  it 
brings  with  it  a  cold  northwest  wind,  carrying  the  temperature  an  equal 
amount  below  the  normal.  The  crests  usually  follow  t!ie  troughs  within 
24  to  36  hours;  they  may,  if  they  are  associated  with  a  rapidly  moving 
storm,  follow  one  another  at  intervals  of  13  hours,  or  even  less. 

In  the  winter  season  conditions  are  favorable  in  the  United  States  for 
bringing  about  great  fluctuations  in  temperature  in  short  periods  of 
time.  Just  to  the  south  we  have  the  tropical  regions  where  temperatures 
are  high  throughout  the  year,  and  atmospheric  moisture  abundant. 
Beyond  our  nortliern  boundary  line  are  the  regions  of  long  winter  nights 
and  a  clear  dry  atmospliere,  factors  favoring  a  rapid  lowering  of  tempera- 
ture by  radiation.  Warm  and  cold  air  are  alternately  brought  to  the 
regions  midway  between  these  reservoirs  of  heat  and  cold,  as  areas  of 
low  and  high  atmospheric  pressure,  or  cyclones  and  anti-cyclones,  follow 
one  another  in  rapid  succession  from  west  to  east  across  tlie  continent. 
This  material  transfer  of  warm  air  by  southerly  winds  and  cold  air  by 
northwest  winds  is  responsible  for  fluctuations  of  great  magnitude.     The 


402 


THE    CLI]\rATE    OF   BALTIMORE 


Fig.  142.— Cold    February    11,    1899. 


Fig.  143. — Warm  February  11,  1887. 


MARYLAND   WEATHER   SERVICE  403 

contrast  is,  in  all  well  developed  cold  waves,  intensified  by  conditions 
attending  all  cyclones  and  anti-cyclones.  The  warm,  moist  southerly 
winds  are  attended  with  cloudy  skies  which  cut  off  rapid  terrestrial  radia- 
tion, thus  preventing  loss  of  heat.  The  cold  northwest  winds  are  dry, 
while  the  sky  is  generally  clear,  permitting  rapid  loss  of  heat  by  terres- 
trial radiation.  These  processes,  attending  all  storms,  are  sufficient  to 
account  for  the  greatest  fluctuations  observed  in  terrestrial  temperatures, 
making  it  entirely  unnecessary  to  call  in  the  aid  of  exti-a-terrestrial  forces 
to  explain  any  unusually  high  or  low  temperatures  observed. 

The  great  fluctuations  in  temperature  experienced  in  past  years  in 
Baltimore  are  discussed  at  considerable  length  in  the  preceding  pages  of 
this  report  (see  especially  Plate  lY,  following  page  82)  ;  the  conditions 
under  which  they  occurred,  however,  are  not  described.  Curve  D,  Plate 
IV,  shows  the  extreme  range  of  temperature  for  each  day  of  the  year 
in  a  period  of  30  years.  The  great  ranges  are  shown  to  be  most  frequent 
in  the  winter,  months,  although  the  month  of  March  is  not  far  behind 
in  this  respect.  February  11  shows  the  greatest  range — on  the  11th  of 
February,  1887,  a  maximum  of  72°  was  recorded  in  Baltimore;  on 
February  11,  1899,  a  minimum  of  6°  below  zero,  making  a  total  range  for 
the  11th  of  February  of  78°.  Even  in  the  month  of  least  variability, 
August,  the  difference  between  the  observed  maximum  and  minimum 
is  31°. 

The  general  weather  conditions  which  prevailed  upon  the  two  days 
of  February,  showing  such  great  contrasts  in  temperature  are  represented 
in  the  accompanying  charts.     (See  Figs.  142  and  143.) 

On  the  11th  of  February,  1887,  a  well  developed  cyclone  was  passing 
eastward  with  its  center  almost  over  Baltimore.  Warm  southerly  winds 
had  been  blowiEg  over  the  city  for  a  considerable  period,  and  with  some 
force.  By  the  afternoon  of  the  11th  the  temperature  had  risen  to  50°. 
The  depression  was  followed  closely  by  an  anti-cyclone,  and  on  the  fol- 
lowing days  the  temperature  fell  to  a  minimum  of  23°  as  the  center  of 
the  anti-cyclone  approached. 

On  the  11th  of  February,  1899,  Baltimore  was  within  one  of  tlie  most 
extensive   and   intense  anti-cyclonic   areas   recorded    in    local    weather 


404  THE    CLIiLA.TE   OF   BALTIMORE 

chronolog}',  two  days  before  the  great  blizzard  of  this  month.  The  cold 
northwest  winds,  aided  by  the  intense  terrestial  radiation  permitted  by 
the  clear  skies  and  dry  atmosphere,  lowered  the  early  morning  tempera- 
ture to  6°  below  zero,  within  a  degree  of  the  greatest  cold  recorded  in 
Baltimore  in  30  years. 

Even  more  striking  are  the  fluctuations  in  temperature  attending  the 
passage  of  single  storms.  On  the  24th  of  February,  1900,  Baltimore 
was  within  the  area  of  influence  of  a  cyclone,  followed  closely  by  an 
energetic  anti-cyclone.  By  7  p.  m.,  with  strong  southerly  winds,  the 
temperature  rose  to  55°.  About  8  p.  m.  the  wind  suddenly  changed  to 
a  strong  northwest  wind  and  the  temperature  fell  rapidly  to  8°  by  mid- 
night, a  fall  of  47°  in  five  hours. 

HOURLY  CHANGES  ON  FEBRUARY  24,  1900. 

P.  M.  Mid- 

Xoon.     1        2       3       4        5       6  7       8  9  10  11  night. 

Pressure  Unches) 29.43    .36    .31    .23    .19    .19    .18  .19    .18  .18  .19  .20    29.21 

Temperature  (F.) 43      45      46      46      49      61      62  55      37  33  28  18         8 

Wind  Direction SE      E    SE    SE    SE      8    SW  SW  NW  W  NW  W     NW 

Wind  Velocity  (miles  per  hour)         8       8       7     12       8       7      10  12     12  6  8  7         8 

On  the  31st  of  December,  1898,  as  the  center  of  a  depression  passed 
over  Baltimore  and  was  followed  by  the  advancing  front  of  an  anti- 
cyclone, the  temperature  fell  from  57°  at  7  a.  m.  to  18°  at  midnight. 
The  usual  diurnal  rise  in  temperature  to  3  p.  m.  was  totally  obliterated. 
The  temperature  continued  to  fall  steadily  to  a  minimum  of  5°  by 
8  a.  ra.  of  January  2,  1899. 

HOURLY  TEMPERATURE  CHANGES  OF  DECEMBER  31,  1898. 

A.  M.  P.  M.  Mid- 

7       8       9       10      n  Noon.  1  3  3  4     5     6     7  8  9  10  11  night. 

Temperature  (F.)..   57     57      55      47     41  38  38  38  37  35  33  31   29  37  25  22  20       18 

WindDirection SW  SW  NW  NW  N  N  N  N  N  N    N    N    N  N  N  N  N        N 

The  Weather  of  Christmas  Day  {December  i;i5). 

In  order  to  illustrate  the  variability  of  wanter  weather  in  Baltimore, 
special  days  have  been  selected  and  the  general  conditions  on  that  day 
for  a  long  series  of  years  graphically  represented  upon  a  single  page. 


MARYLAND   WEATHER   SERVICE  405 

As  the  popular  interest  in  the  weather  conditions  upon  public  holidays 
is  always  great,  one  or  two  days  have  been  selected  in  each  season.  For 
the  winter  season  we  have  chosen  Christmas  Day  and  Washington's 
Birthday.  Charting  the  weather  conditions  in  this  manner  the  contrasts 
are  more  readily  perceived  than  b}'  the  use  of  words  and  figures. 

The  factors  represented  are:  The  maximum,  minimum,  and  mean 
temperatures  for  the  day,  the  mean  atmospheric  pressure,  the  average 
amount  of  cloudiness  during  the  course  of  the  day,  the  prevailing  wind 
direction,  and  the  amount  of  precipitation. 

Take  for  example  the  weather  conditions  experienced  upon  the  25th 
of  December  in  each  year  from  ISTl,  the  date  of  establishment  of  the 
local  office  of  the  United  States  Weather  Bureau  in  Baltimore,  to  the 
year  1906. 

The  mean  temperature  for  the  month  of  December  is  37° ;  for  the 
25th  of  December  it  is  35.5°.  The  maximum  temperature  on  the  25th 
was  on  two  occasions  (in  1889  and  in  1893)  as  high  as  67°;  in  1872 
the  minimum  temperature  of  the  day  was  8°,  a  range  of  59°.  The 
fluctuations  from  year  to  year  are  very  irregular;  sometimes  they  are 
abrupt,  as  the  change  from  1871  (44°)  to  1872  (14°)  ;  at  other  periods 
they  are  gradual,  as  from  1881  to  1884,  a  slight  and  steady  fall  from 
40°  to  28°  in  the  mean  temperature  of  the  day.  There  were  9  clear 
days,  13  fair  or  partly  cloudy  days,  and  14  cloudy  days.  The  winds 
were  prevailingly  east  to  northeast  and  but  once  from  the  south. 

The  day  has  been  remarkably  free  from  precipitation  of  measurable 
quantities,  and  this  has  been  mostly  in  the  form  of  rain.  In  not  a  single 
year  since  1871  has  snow  fallen  to  the  depth  of  one  inch  upon  Christmas 
Day.  Snow  has  been  on  the  ground  to  a  greater  depth  but  in  such 
instances  it  fell  on  the  day  preceding  and  remained  on  the  ground.  (See 
Fig.  144.) 


:§    ^ 


O 

ts. 

O 

g 

o 

C 

o 

fa 

c 

C 

— 

C 

•     c 

5 

. 

U 

;    1 

\  A^  ■ 

■  a~i      ; 

,^;; 

'9 

^ 

^i^^' 

'  5    i 

o 
o 

ti 

\ 

1 

<    T    . 

>i 

i 

£ 

N; 

^> 

>> 

\ 

z 

►  ^ 

1 

i  "-J    : 

.  en 

;  —1     ; 

k 

'/ 

/ 

2: 

CO 

5' 

i| 

^ 

si 

"CC 

oz     f    : 

UJ 

La 

Si 

:  2 

m 

}< 

/ 

!  ,^^  :    4 

~Q 

in 

<*?- 

;^ 

H 

£ 

:\ 

^V) 

GO 

4^ 

< 

;  vl!/  :  ^ 

^ 

^^ 

5»i_ 

S 

^ 

J 

^ 

•^ 

-> 

1  f 

> 

/ 

o 

05 

r' 

% 

^ 

■  ffi  '  ^ 

f 

JD 

- 

i^ 

■"^ 

'^' 

f 

X 

\ 

V 

^■ 

^x 

^ 

1    1 

i 

rs 

[i 

I 

S  : 

\/ 

\: 

:. 

': 

\ 

^ 

k 

I 

/: 

: 

1 

cc 

t 

^ 

/ 

V 

.' 

fe 

'A 

\ 

y 

// 

/ 

■ 

; 

i  w  '■    ^ 

' 

1  >. 

n 

-*\ 

I 

i  /'^  ^ 

o 

1 

\^' 

s 

N 

\ 

i 

! 

i 

^  r1 

/ 

■ 

; 

V 

J. 

' 

i 

"^ 

^ 

vJ 

i 

or 

! 

;  uj 

'0 

^^ 

^ 

; 

'^ 

y 

i:-iiX-Li 

Irt 

:  ^ 

y^ 

?^ 

f 

si. 

= 

^i 

ac 

i 

— ^ 

^ 

> 

: 

s; 

'■■ 

r 

\ 

^ 

^ 

■^^ 

,/ 

t 

^^ 

-^ 

^ 

^ 

: 

ii     • 

i  I  ' 

*. 

1  tr 

u. 

; 

'i  1  ; 

\ 

i  V; 

k 

15     ; 

1 

o   ^ 


50 

—  M 


o      in     o      «rt     c> 
{vi     —     —      o 


MAKTLAXD   WEATHER   SERVICE 


407 


THE    CHARACTER    OF    THE    WEATHER    ON    CHRISTMAS    DAY    IN    BALTIMORE 

FROM  1871-1906. 


Year. 

Max. 
Temp. 

Min. 
Temp. 

Mean 
Temp. 

Character 
of  Day. 

Wind 
Direction. 

Daily  Wind 
Movement. 

Precip- 
itation.i 

(De 

i-rees  Fah 

r.) 

(Miles) 

(Inches) 

1871 

48 

40 

44 

Pt.  cldy 

NE 

45 

1872 

19 

8 

14 

Cloudy 

NE 

159 

1873 

41 

30 

36 

Pt.  cldy 

SE 

98 

1874 

39 

30 

34 

Clear 

W 

245 

1875 

54 

40 

47 

Pt.  cldy 

E&S 

52 

0.09 

1876 

27 

18 

22 

Cloudy 

N&NE 

85 

0.06 

1877 

47 

43 

45 

Cloudy 

E 

58 

0.03 

1878 

25 

15 

20 

Clear 

W 

268 

1879 

52 

32 

42 

Cloudy 

N 

95 

0.38 

1880 

38 

25 

32 

Cloudy 

NE 

111 

0.35 

1881 

49 

32 

40 

Clear 

W 

72 

1882 

47 

36 

42 

Pt.  cldy 

w 

108 

1883 

40 

32 

36 

Pt.  cldy 

NE&SW 

70 

0.39 

1884 

32 

25 

28 

Cloudy 

N 

191 

1885 

39 

30 

34 

Pt.  cldy 

NE 

144 

1886 

45 

28 

36 

Clear 

N&NW 

198 

1887 

38 

31 

35 

Cloudy 

N 

106 

1888 

53 

29 

41 

Clear 

Calm 

17 

1889 

67 

44 

56 

Pt.  cldy 

SW 

101 

0.04 

1890 

32 

25 

28 

Cloudy 

NE&NW 

114 

1891 

48 

47 

48 

Cloudy 

SE 

127 

0.02 

1892 

29 

14 

22 

Cloudy 

NW 

225 

0.02 

1893 

67 

40 

54 

Clear 

SW 

116 

1894 

48 

38 

43 

Pt.  cldy 

NW 

171 

0.15 

1895 

52 

43 

48 

Cloudy 

E 

122 

0.01 

1896 

28 

18 

23 

Clear 

W 

106 

1897 

34 

16 

25 

Clear 

S 

62 

1898 

38 

32 

35 

Pt.  cldy 

NE 

76 

1899 

35 

23 

29 

Pt.  cldy 

W 

134 

1900 

50 

35 

42 

Pt.  cldy 

W 

115 

1901 

48 

36 

42 

Cloudy 

NE 

85 

1902 

32 

19 

26 

Pt.  cldy 

W 

165 

0.17 

1903 

43 

34 

38 

Cloudy 

NW 

83 

0.19 

1904 

28 

23 

26 

Cloudy 

NE 

194 

0.14 

1905 

38 

26 

32 

Pt.  cldy 

NW 

162 

1906 

33 

13 

22 

Clear 

NW 

299 

Means 

42 

30 

36 

Pt.  cldy 

142 

0.14 

•  Amounts  loss 

than   .01    inch 

not  considered. 

27 


c         ^^ 

O 

c 

o  ^6 

o' 

rt 

CvJ    — « 

(?>        o 


w      —     — 


.^ 

■'";■■■■     ■ 

: 

; 

! 

\    }  ' 

:/+\  i 

:  oo      : 

: 

) 

I 

h- 

.  \ 

\: 

;i  i 

V^^ 

:5= 

^ 

! 

/! 

>-  \-^  • 

<     I     ■• 

•  ■ 

■ 

■  i, 

1^ 

/ffij 

• 

7!  1   ^ 

>- 
a: 

N 

yH 

o  \J 
cc     1 

1^ 

C3 

5- 

[ 

ii 

TEMPERATURE 

ki-^ 

^ 

/ 

4/ 

i. 

•\  ^ 

UJ 

u. 

; '' 

K 

K 

? 

\ 

!      £ 

; 

/| 

\: 

I 

;   J^  ;    , 

j 

Y 

^ 

^ 

s.  ^ 

/X  :  , 

i 

i 

> 

^ 

^ 

/ 

; 

i 

\\ 

M 

1 

'■ 

M  : 

/ 

v\ 

s^ 

: 

•  r^  ■     1 

s 

%l 

/ 

(T^^  *■ 

: 

^ 

7; 

i 

^  ■ 

'^■ 

; 

\ 

,_ 

\ 

^^ 

v 

\ 

:     * 

^ 

=^ 

< 

/ 

n\ 

f 

"^ 

^ 

V 

():  y 

/ 

: 

\  ^ 

'^U 

i\ 

1 

\    ; 

y 

i\ 

h 

/■\ 

1 

TEMPERATURE" 

/ 1 

:| 

; 

^ 

\[ 

^ 

\ 

: 

^=^- 

■    •-Tn    ■ 

\\ 

f^ 

viks 

/ 

;o^ 

/'  \ 

^\ 

f 

1   c= 

1     =3 

_v-/ 

o  V 

UJ  •*• 

-  :i:-fc» 
o  /"^ 

^-  1 

i:   \ 

\ 

\  i 

/ 

Q: 

/I: 

1 

2 

—  a:" 

<: 

.> 

^ 

J::^ 

■^ 

\ 

i: 

"^ 

^ 

^: 

-^ 

St 

J: 

1 

7 

./ 

§  V 

r- 

\ 

\^ 

i^ 

y 

^  J 

i 

T-^  : 

^__ 

: 

CQ 


MARYLAND   WEATHER   SERVICE 


409 


The  Weather  of  Washington's  Birthday  (February  22). 
The  weather  of  the  month  of  February  presents  more  contrasts  than 
that  of  any  other  portion  of  the  year.  The  22d  is  no  exception ;  with 
an  average  of  38°  the  temperature  was  as  high  as  74°  in  1874,  and  as 
low  as  13°  in  1896,  a  range  of  61°.  From  1878  there  was  a  steady  fall 
in  the  mean  temperature  of  the  day  to  the  year  1885,  though  there  is 
apparently  no  regular  period  discernible  in  the  annual  fluctuation. 
The  grouping  of  the  days  with  precipitation  is  somewhat  striking.  From 
1871  to  1890  there  were  but  two  occasions  upon  which  rain  or  snow 
fell  to  any  considerable  depth ;  namel}-,  in  1876  with  a  rainfall  of  three- 
tenths  of  an  inch,  and  IS 78  with  a  heavy  rainfall  measuring  over  an  inch 
and  seven-tenths.  Snow  fell  in  1879,  1882,  1883,  and  in  1889,  but  the 
fall  was  extremely  light  in  all  cases.  From  1891  to  1907  rain  or  snow 
fell  in  1891,  1893,  1894,  1897,  1900,  1901,  1902,  and  in  1904,  though 
the  amounts  were  small  in  1892,  1898,  1899,  1903,  and  1907.    Fig.  145. 


THE    WEATHER    OF    FEBRUARY    22. 


Year. 

Max. 
Temp 

Min. 
Temp. 

Mean 
Temp. 

Character 
of  Daj'. 

Wind 
Direction. 

Daily  Wind 
Movement. 

Precip- 
itation. 

(Degrees  Fab 

r.) 

(Miles) 

(Inches) 

1871 

35 

20 

28 

Clear 

W 

.    .    . 

.  .  . 

1S72 

35 

28 

32 

Clear 

NW 

170 

1873 

35 

24 

29 

Pt.  cldy 

•     NW 

199 

1874 

74 

51 

62 

Pt.  cldy 

SW 

173 

1875 

48 

29 

38 

Pt.  cldy 

E 

97 

1876 

46 

36 

41 

Clear 

W 

166 

0.34 

1877 

57 

30 

44 

Pt.  cldy 

SE 

66 

1878 

63 

50 

56 

Cloudy 

W 

196 

1.71 

1879 

36 

21 

28 

Cloudy 

S 

167 

0.05 

1880 

50 

31 

40 

Clear 

S 

110 

1881 

47 

32 

40 

Clear 

SE 

143 

1882 

40 

34 

37 

Pt.  cldy 

NW 

279 

1883 

43 

31 

37 

Cloudy 

SW 

95 

0.01 

1884 

49 

36 

42 

Pt.  cldy 

SE 

90 

1885 

27 

14 

20 

Clear 

NW 

183 

1886 

43 

32 

38 

Clear 

S 

168 

1887 

54 

35 

44 

Pt.  cldy 

N 

158 

1888 

56 

35 

46 

Clear 

N 

74 

1889 

36 

30 

33 

Cloudy 

SW 

81 

0.09 

1890 

44 

25 

34 

Clear 

w 

127 

410 


THE    CLIMATE    OF   BALTIMORE 


Year. 

1891 

Max.           Min.          Mean 

Temp.       Temp.       Temp. 

(Degrees  Fahr.) 

47             38             42 

Character 
of  Day. 

Clear 

Wind 
Direction. 

NW 

Daily  Wind 
Movement. 
(Miles) 
203 

Precip- 
itation. 
(Inches) 
0.54 

1892 

49 

39 

44 

Pt.  cldy 

NE 

260 

1893 

32 

24 

28 

Clear 

W 

524 

0.21 

1894 

35 

27 

31 

Clear 

W 

109 

0.32 

1895 

32 

29 

30 

Pt.  cldy 

NW 

300 

1896 

42 

13 

28 

Clear 

sw 

180 

1897 

43 

36 

40 

Cloudy 

sw 

201 

0.71 

1898 

43 

34 

38 

Cloudy 

w 

111 

1899 

60 

42 

51 

Cloudy 

w 

122 

1900 

57 

41 

49 

Pt.  cldy 

w 

164 

0.54 

1901 

34 

21 

28 

Pt.  cldy 

w 

120 

0.01 

1902 

36 

34 

35 

Cloudy 

N 

224 

0.40 

1903 

34 

25 

30 

Clear 

NW 

227 

1904 

47 

34 

40 

Pt.  cldy 

NW 

240 

0.74 

1905 

39 

31 

35 

Cloudy 

NB 

240 

1906 

54 

39 

47 

Clear 

N 

253 

1907 

26 

16 

21 

Clear 

NW 

249 

Means 


45 


31 


Pt.  cldy 


179 


0.44 


SPKING  WEATHEE. 

The  spring  season  is  a  transition  period  between  winter  and  summer 
weather  conditions.  Normally  the  season  has  three  months,  but  May 
is  practically  a  summer  month,  while  March  frequently  has  more  of  the 
characteristics  of  winter  than  spring.  The  season,  so  far  as  injurious 
weather  conditions  are  concerned,  is  very  brief.  The  cyclones  and  anti- 
cyclones of  early  spring  belong  to  the  winter  type;  they  are  quite  as 
energetic,  and  follow  much  the  same  paths.  With  the  steady  approach 
of  the  sun  the  increased  heat  becomes  more  apparent,  however,  and  the 
contrasts  in  temperature  between  the  winds  preceding  and  following  the 
travelling  cyclones  become  more  marked. 

One  of  the  first  harbingers  of  spring  is  the  appearance  of  an  area  of 
high  atmospheric  pressure  off  the  coast  of  the  South  Atlantic  states. 
This  high  area  may  have  advanced  from  the  northwest  and,  after  crossing 
the  continent,  settled  off  the  coast,  but  it  is  more  likely  to  be  an  inde- 


MARYLAND   WEATHER   SERVICE  411 

pendent  formation  in  place,  or  the  westward  extension  of  the  permanent 
area  of  high  pressure  normally  found  over  the  Atlantic  Ocean  between 
the  Azores  and  the  South  Atlantic  states.  The  presence  of  an  anti- 
cyclone in  the  southeast  gives  to  the  Middle  Atlantic  states  a  steady  flow 
of  warm  southeast  to  southwest  winds. 

March  weather  is  extremely  variable.  The  month  has  given  us  some 
of  our  severest  winter  weather,  such  as  the  blizzard  of  1888,  described 
in  preceding  pages;  it  may  also  be  excessively  cold  and  raw,  as  in 
1906.  On  the  other  hand,  there  may  be  an  abundance  of  fine  warm 
days  forcing  all  vegetable  growth  four  or  five  weeks  in  advance  of  the 
average  season,  as  in  March,  1898. 

The  explanation  for  these  striking  contrasts  may  be  found  in  the 
general  distribution  of  atmospheric  pressure  over  the  North  American 
continent  and  adjacent  oceans  during  the  early  spring  season.  Under 
normal  conditions  in  the  month  of  March  there  is  a  well  devek)ped  area 
of  high  pressure  over  the  central  portion  of  the  continent — the  British 
Northwest  Territory;  another  area  of  high  pressure  prevails  over  the 
Atlantic  Ocean  with  its  axis  along  the  parallel  of  30°  north  latitude, 
extending  westward  nearly  to  the  Atlantic  coast.  These  areas  are  not 
the  same  as  the  travelling  anti-cyclones  described  in  the  preceding  pages. 
They  remain  nearly  stationary  for  long  periods  of  time,  but  are  sub- 
ject to  more  or  less  shifting  about  a  central  point  from  time  to  time. 
The  travelling  anti-cyclones  are  probably  detached  portions  of  the  larger 
areas.  The  character  of  the  weather  within  these  so-called  permanent 
areas  of  high  pressure  is  the  same,  however,  as  is  found  within  the 
smaller  travelling  anti-cyclones.  Normally,  the  Middle  Atlantic  states 
lie  in  a  belt  between  these  two  high  areas,  and  alternately  fall  within 
the  influence  of  first  one,  then  the  other.  One  brings  us  cold  weather, 
the  other  warm.  Occasionally  the  continental  high  area  will  extend 
to,  or  move  southward  and  eastward  of,  its  normal  limits  and  bring 
within  its  influence  the  whole  of  the  Middle  Atlantic  states.  Such  was 
the  case  in  the  years  1883,'  1885,  1888,  and  1801.     Tlie  month  of  :March 

^8ee:  0.  L.  Fassig.  Types  of  March  Weather  in  the  U.  S.  Amer.  Jour. 
Soi.,  Nov.,  1899. 


412  THE    CLIMATE    OF   BALTIMORE 

in  these  years  was  decidedly  below  the  normal  in  temperature  throughout 
the  Middle  i\.tlantic  states.  Again  there  may  be  a  weak  development 
of  the  continental  high  area,  in  conjunction  with  a  strong  development 
of  the  Atlantic  Ocean  high  area,  or  its  extension  westward  beyond  its 
usual  limits.  The  Middle  Atlantic  states  would  then  be  brought  within 
its  influence  and  produce  strong  southeast  to  southwest  winds  or  light 
variable  winds  and  high  temperatures.  During  the  month  of  March 
in  1878,  1882,  1894,  and  1898  the  distribution  of  the  monthly  mean 
pressure  was  such  as  is  indicated,  and  in  all  of  these  months  the  tempera- 
ture was  well  above  the  normal  value.     (See  Plate  XXIV.) 

March  Winds  and  Storms. 
March  is  proverbally  a  windy  month.     The  wind  movement  for  this 
month  is  the  largest  of  the  year,  exceeding  even  that  of  the  winter 
months : 

AVERAGE  DAILY  WIND  MOVEMENT  AT  BALTIMORE. 
(Average  of  30  years.) 
Jan.    Feb.      Mar.     Aiir.     May.  June.  July.  Aug.    Sept.    Oct.    Nov.   Dec.  Year. 
Miles 143       163         175         166         119       143        134       122         129        137       143       143       145 

This  comparatively  high  wind  velocity  is  probably  due  to  the  great 
contrasts  in  temperature,  characteristic  of  the  month.  It  is  the  time  of 
the  year  when  the  temperature  rises  most  rapidly  and  the  inter-diurnal 
changes  in  temperature  are  greatest: 

MEAN    DAILY    CHANGE    IN    TEMPERATURE    AT    BALTIMORE. 

(Based  on  30  years  of  observations.) 

Jan.    Feb.     Mar.     Apr.     May.    June.   July.     Aug.    Sept.     Oct.    |Nov.     Dec. 
Av.  daily 
change±  1.0°      1.0°       1.2°       0.8°       1.0°        0.6°       0.6°         0.4°       0.9°       0.:°        1.0°    0.8°  F. 

March  follows  February  in  the  frequency  and  duration  of  storm  winds : 

AVERAGE    DURATION    AND   FREQUENCY    OP    STORM    WINDS    AT    BALTIMORE. 
(Average  of  5  years'  observations.) 


Average  monthly  f  requencj- ) 

of  winds  exceeding25  miles  >    6.3     9.3     7.6 

per  hour.  ) 

Average  duration  in  hours  '    o  i-n     . 

and  minutes.  \    ^■''"    * 

Longest  periods  of  continu- I     ,„ 

ous  storm  winds  (hoursj.     i' 


fe 

< 

a 

>^ 

ic 

o 

O 

o          ® 

9.3 

7.6 

5.0 

4.4 

2.0 

2.6 

1.1 

3.6 

4.3 

■4.8     4.4 

1.40 

3.40 

3.30 

1.30 

0.13 

0.30 

3.00 

1.25 

3.40 

2.00   3.00 

46 

23 

16 

7 

0.5 

0.5 

IS 

7 

9 

6       19 

MARYLAND  WEATHER    SERVICE. 


VOLUME   2,   PLATE  XXM. 


EFFECTS  OF  THE  ICE-STORM  OF  MARCH,  1906    IN  THE  BLUE  RIDGE 

MOUNTAINS. 


MARYLAND   WEATHER    SERVICE  413 

ICE    STORilS. 

While  the  afternoon  temijeratures  during  March  are  well  above  the 
freezing  point,  the  early  morning  temperatures  generally  dip  below  the 
frost  line.  Hence  the  isotherm  of  32°  is  frequently  crossed  during  the 
passage  of  the  storms  of  this  month.  Precipitation,  beginning  as  rain, 
changes  to  sleet  or  snow,  or  the  rain  as  it  falls  upon  the  cold  ground 
or  trees  and  shrubs  freezes,  covering  all  exposed  objects  with  a  coating  of 
ice.  These  ice  storms  are  of  frequent  occurrence  in  the  early  spring, 
but  generally  occur  in  March.  They  frequently  cause  great  damage  to 
property  by  overloading  trees,  telegraph  lines,  etc.,  and  are  a  source  of 
considerable  danger  to  pedestrians  on  the  city  streets.  The  ice  upon  trees 
and  telegraph  lines  sometimes  collects  in  great  quantity.  Under  favor- 
able conditions  in  the  presence  of  a  fog  or,  in  the  mountain  districts 
in  a  driving  low  cloud,  the  frozen  particles  of  fog  or  cloud  collect  upon 
the  windward  side  of  exposed  objects  to  a  thickness  of  two  and  even 
three  inches.     (See  Pis.  XXI  and  XXII.) 

THE  SQUALL  OF  MARCH   1,  1907. 

A  type  of  storm  which  occurs  with  increasing  frequency  with  the 
advance  of  spring  is  shown  on  the  weather  map  of  March  1,  1907.  The 
eastward  advance  of  the  storm  across  the  country  is  marked  by  the  occur- 
rence of  severe  squalls  and  thunderstorms,  accompanied  by  heavy  rains, 
within  a  restricted  area  of  the  general  storm,  or  cyclone.  The  isobars 
enclosing  the  storm  area  form  long  narrow  troughs  which  are  likely  to 
be  particularly  well  marked  as  the  cyclone  passes  across  the  Mississippi 
Valley.  Drawing  an  approximately  north  and  south  line  through  the 
center  of  the  storm  the  winds  to  the  east  blow  from  thu  southeast,  while 
those  to  the  west  blow  from  the  northwest.  In  the  vicinity  of  this  line 
of  conflict  between  the  southeast  and  northwest  winds,  from  the  center 
of  the  storm  southward,  we  have  numerous  squalls,  thunderstorms,  and 
heavy  rains.  These  local  storms  occur  almost  entirely  within  the  south- 
ern quadrant  of  the  general  cyclone  of  the  type  here  described.  The 
contrasts  in  temperature  between  the  southeast  winds  to  the  east  of  the 
"squall   line,"   as   the   north-south  line   above   described   is  sometimes 


414 


THE    CLIMATE    OF   BALTIMORE 


calltjd,  and  the  northwest  winds  to  the  west,  are  very  pronounced;  these 
contrasts  are  especially  well  marked  in  passing  from  the  center  of  the 
storm  in  a  northwesterly  direction.  The  map  of  March  1,  1907,  shows 
a  difference  of  50°  at  8  a.  m.  between  St.  Louis,  Mo.,  and  Huron,  S. 
Dak.,  a  distance  of  about  500  miles.  The  chart  also  shows  the  distribu- 
tion of  thunderstorms,  occurring  within  the  12-hour  period  preceding  8 


Fig.  146.— The  Squall  of  March  1,  1907. 


a.  m.  of  this  date.  The  rains  of  the  Lower  Mississippi  Valley  were 
heavy,  and  in  some  localities,  excessive:  Mobile,  Ala.,  reported  a  fall 
of  6.42  inches  in  the  preceding  24  hours;  Montgomery,  Ala.,  1.32  inches; 
Anniston,  Ala.,  1.16  inches;  Meridian,  Miss.,  2.64  inches;  Little  Eock, 
Ark.,  1.42  inches;  and  Memphis,  Tenn.,  1.30  inches.     (See  Fig.  146.) 

Later  in  the  season,  with  the  increasing  heat  of  spring,  the  most  intense 
variety  of  local  storm — the  tornado — is  frequently  developed  within  the 
thunderstorm  and  squall  area  of  this  type  of  general  storm.     As  the 


MARYLAND   WEATHER   SERVICE  415 

storm  moves  farther  eastward  the  squalls  and  thunderstorms  continue 
to  develop,  but  the  tornado  becomes  of  less  frequent  occurrence,  disappear- 
ing almost  entirely  by  the  time  the  center  of  the  general  storm  reaches 
the  coast.  In  Maryland,  for  example,  a  real  tornado  is  of  very  rare 
occurrence. 

The  storm  described  above  changed  its  shape  materially  during  the 
succeeding  24  hours,  becoming  by  8  a.  m.  of  March  2  more  circular  in 
form,  with  its  center  over  the  Lower  Lake  region.  In  changing  to  the 
more  common  cyclonic  type  the  change  in  wind  direction  during  the 
passage  of  the  center  of  the  storm  became  less  abrupt  and  the  squally 
character  of  the  shift  in  the  wind  disappeared  to  a  great  extent. 

EQUINOCTIAL   STORMS. 

There  is  a  widespread  popular  belief  in  the  occurrence  of  severe  storms 
at  the  equinoctial  periods  in  March  and  September.  Just  why  there 
should  be  any  unusual  atmospheric  disturbance  when  the  sun  "  crosses 
the  line  "  has  never  been  made  clear.  The  belief  is  an  old  one,  and  is 
especially  firmly  fixed  in  the  minds  of  sailors.  Like  many  of  the  old 
weather  "  saws,''  it  will  not  stand  the  test  of  rigid  comparison  with  re- 
corded observations.  As  a  rule,  people  are  not  very  critical  in  their  defini- 
tions of  storms,  or  in  verifying  the  time  of  their  occurrence.  In  the  ab- 
sence of  a  severe  storm  a  very  mild  disturbance  will  satisfy  them.  Or  if  the 
storm  should  occur  three  or  four  days  preceding  or  following  the 
equinoxial  day  it  is  an  equinoxial  storm  ahead  of  time  or  a  little  delayed 
in  its  arrival.  With  such  elastic  restrictions  it  is  not  difiicult  to  realize 
an  equinoctial  storm  in  any  month  of  March.  But  under  these  condi- 
tions a  storm  is  just  as  likely  to  occur  upon  any  other  day  in  the  month. 
If  a  disturbance  be  required  to  show  a  wind  velocity  exceeding  25  miles 
per  hour  in  order  to  be  classed  as  a  storm,  there  is,  according  to  the 
Baltimore  records,  a  storm  wind  every  fourth  day.  If  uniformly  dis- 
tributed through  the  month  any  clay  might  be  selected  for  a  storm  and 
an  error  of  more  than  two  days  in  time  could  not  be  made.  Moreover, 
the  records  show  that  the  wind  velocities  on  the  21st  and  22d  of  Marcli 
are  not  greater  than  on  the  days  preceding  and  following.     If  we  con- 


416  THE    CLIMATE    OF   BALTIMORE 

sider  the  occurrence  of  gales,  or  winds  exceeding  40  miles  per  hour,  we 
find  that  during  a  period  of  30  years  there  were  42,  distributed  through 
the  year  as  indicated  below : 

FREQUENCY  OF  GALES   NEAR   BALTIMORE. 

Jan.      Feb.      Mar.     Apr.     Maj\     June.     July.      Au^.      Sept.      Oct.      Nov.    Dec.    Year. 
3  10  3  2  2  3  4  3  0  3  5  6  42 

September,  the  month  of  the  autumnal  equinoctial  storm,  is  the  only 
month  in  the  year  without  a  gale  to  its  credit  in  30  years,  while  the 
month  of  March  has  less  than  the  average  monthly  frequency. 

If  we  regard  rainfall  as  one  of  the  essential  features  of  a  storm,  statis- 
tics are  no  more  favorable  to  the  equinoctial  theory  than  they  are  in 
the  case  of  winds.  On  March  21  the  rainfall  frequency,  based  on  31 
years  of  observations,  is  slightly  less  than  50  per  cent;  that  is,  in  31 
years,  rain  occurred  15  times;  on  the  22d  the  percentage  of  frequency 
is  40  per  cent.  On  these  days  rain  has  occurred  less  than  half  the  time. 
Eainfall  frequency  is  somewhat  greater  on  the  19th  and  20th,  having 
occurred  16  times  in  31  years.     (See  Table  XLIV,  page  181.) 

When  considering  amount  of  rainfall  instead  of  frequency,  statistics 
are  even  more  unfavorable.  The  average  amount  of  rainfall  recorded 
in  Baltimore  during  30  years  on  the  21st  of  March  is  0.21  inch.  The 
amounts  for  March  19  and  20  are  decidedly  greater,  namely,  0.45  inch 
and  0.37  inch  respectively.  The  daily  average  for  the  entire  month  is 
0.31  inch;  hence  the  amount  recorded  on  the  21st  is  below  the  average 
for  the  month. 

Similar  statistics  may  be  shown  for  the  September  equinoctial  day : 
On  September  21,  the  average  rainfall  is  0.16  inch;  for  the  19th  it  is 
0.93  inch,  nearly  six  times  as  much;  for  the  20th  it  is  0.23  inch;  for  the 
22d,  0.30  inch.  The  average  daily  amount  for  the  entire  month  of 
September  is  0.30  inch.     For  rainfall  frequency  in  September  we  have: 

September  19 26  per  cent. 

20 30 

21 26 

22 32 


MARYLAND   WEATHER    SERVICE  417 

The  daily  average  for  the  entire  montli  is  30  per  cent.  Eain  occurs 
on  the  21st  day  of  March  on  the  average  in  approximately  alternate 
years;  on  the  21st  day  of  September  every  third  year.  The  equinoctial 
storm,  as  a  destructive  storm,  or  as  a  storm  confined  to  a  single  day  in 
the  vicinity  of  Baltimore,  is  a  myth. 

Hail  Storms. 

True  hail  is  of  infrequent  occurrence  in  the  winter  months.  While 
it  is  often  reported  in  the  cold  season  a  careful  observer  would  in  nearly 
every  instance  report  sleet.  There  is  a  radical  difference  between  sleet 
and  hail,  both  in  the  manner  of  formation  and  in  physical  character- 
istics. Sleet  is  an  intermediate  stage  between  rain  and  snow,  or  a  mix- 
ture of  the  two  forms,  and  occurs  during  the  passage  of  a  storm  when 
the  temperature  crosses  the  freezing  point.  Hail,  on  the  other  hand, 
occurs  almost  entirely  during  the  warm  season,  when  the  surface  tem- 
peratures are  far  above  the  freezing  point  of  water. 

THE  FREQUENCY  OF  OCCURRENCE  OF  HAIL  IN  BALTIMORE. 

(Total  number  in   28  years.) 

Jan.      Feb.      Mar.      Apr.      May.     .Tune.     .Tuly.     Aug-.     Sept.     Oct.     Nov.     Dec.    Year 
4  3  3  3  II  13  9  6  1  0  1  1  55 

It  will  be  noted  by  the  above  figures  that  over  half  of  the  hail  storms 
reported  in  Baltimore  in  28  years  occurred  in  the  months  of  May,  June, 
and  July.  Further  details  as  to  frequency  and  time  of  occurrence  may 
be  found  on  pages  284-287  of  this  report. 

The  most  favorable  period  of  hail  formation  appears  to  be  the  latter 
part  of  the  spring  season,  and  the  early  summer.  The  hail  storm,  like 
the  thunderstorm,  is  intimately  associated  with  the  general  cyclone.  It 
occurs  in  the  southern  quadrant  of  the  general  storm,  along  the  "  squall 
line,"  described  in  a  preceding  paragraph.  Hail  storms  occur  almost 
exclusively  in  connection  with  thunderstorms.  The  reverse  of  this  state- 
ment is,  however,  not  true,  while  but  55  hail  storms  are  recorded  in  the 
local  annals  of  the  Weather  Bureau  in  a  period  of  28  years,  there  is  a 
record  of  678  thunderstorms  during  the  same  period. 


418  THE    CLIMATE    OF   BALTIMORE 

THE   HAIL   STORM    OF   MAY    19,    1904. 

On  Ma}^  17  and  IS,  1904,  a  condition  of  unsettled  weather  prevailed 
over  the  country  east  of  the  Mississippi  Eiver.  Cloudy  and  rainy  weather 
accompanied  the  slow  eastward  movement  of  a  shallow  barometric  depres- 
sion from  the  Middle  Mississippi  Valley  to  the  Middle  and  South  Atlantic 
states.  On  the  morning  of  the  18th  the  center  of  depression  was  over 
North  Carolina.  From  the  morning  of  the  18th  to  the  morning  of 
the  19th  the  storm  became  more  limited  in  area  and  moved  rapidly 
northward.  At  8  a.  m.  the  center  was  over  Lake  Huron,  with  an  exten- 
sion southeastward,  forming  a  secondary  depression.  There  Avas  a  well 
defined  line  of  separation  between  southeast  and  westerly  winds,  extend- 
ing from  the  center  of  the  storm  southeastward  across  Central  New 
York,  Eastern  Pennsylvania,  and  Southern  New  Jersey.  During  the 
19th  the  storm  moved  slowly  eastward,  accompanied  by  severe  local 
storms  and  heavy  rains  in  the  Middle  Atlantic  and  New  England  states. 
In  Maryland  the  thunderstorm  was  accompanied  by  hail  between  two  and 
three  o'clock  in  the  afternoon.     (See  Fig.  147.) 

The  local  conditions  prevailing  at  Baltimore  during  the  progress  of 
this  storm  are  shown  in  detail  in  the  accompanying  diagram  (Fig.  148). 

THE   HAIL    STORM    OP   APRIL   27,    1890. 

A  hail  storm  of  unusual  severity  passed  over  the  city  on  the  27th  of 
April,  1890.  A  detailed  account  of  local  changes  appears  in  the  daily 
journal  of  the  office  of  the  Weather  Bureau,  from  which  we  quote  the 
following : 

Dense  fog  prevailed  in  the  morning,  gradually  disappearing  during  the 
forenoon.  The  temperature  rose  rapidly  from  48°  at  8  a.  m.  to  71°  in  the 
afternoon.     Cautionary  southeast  storm  signals  were  displayed  at  10.45  a.  m. 

The  most  destructive  hail  storm  on  record  at  Baltimore  visited  the  city 
this  afternoon  between  3.45  and  4  o'clock.  It  came  from  a  point  between 
west  and  northwest,  and  travelled  in  a  direction  a  little  south  of  east. 

A  half-hour  before  the  arrival  of  the  storm,  a  dense  black  cloud-mass, 
tinged  with  purple  and  green,  was  observed  extending  from  the  western 
horizon  across  the  sky  to  the  northeast,  and  rapidly  approaching.  There  were 
at  this  time  two  or  three  subdued  peals  of  thunder,  following  some  vivid 
flashes  of  lightning  in  the  west.  The  front  of  the  bank  was  in  great  commo- 
tion and.  as  it  approached,  appeared  to  be  preceded  by  a  thin  misty  veil  of 


MARYLAND   AVEATHEE   SERVICE 


419 


o       H,HIGH 


HIGH 


\ 


A< 


LOV 


\  .3 


/^ 

50° 

X.     \^^ 

GH 

\ 

//       U'f    LOVVl 

V l7~^  ^-""^ 

^/J 

^    ->' — — - 

-/: 

- 

^'^  7  0- 


Fig.  147.— The  Hail  Storm  of  May  19,  1904. 


MDT. 


MAY    1  9    1  904 


2  9':  52  WIND    DIRECTION 

V ■/// ^W 

10      10      IS      fe       4       ;{        II       S       8       8       t) 

§^  WIND    MOVEMENT 

ta  Tr      Tr. 


Fin.  148.— The  Hail  Storm  of  May  19.  1904. 


420  THE    CLIMATE    OF   BALTIMORE 

cloud.  There  was  a  sound  like  the  roll  of  musketry,  and  the  storm  burst  sud- 
denly upon  the  city  with  an  almost  deafening  roar  as  the  great  hail  stones 
rained  down  upon  the  tin  roofs  and  crashed  into  the  windows,  not  a  building 
in  the  path  of  the  storm  escaping  damage. 

Several  persons  were  knocked  down  by  the  stones,  and  many,  including  a 
number  of  children,  were  cut  and  bruised.  Horses,  pelted  and  cut  until  the 
blood  streamed  from  them,  could  not  be  controlled,  and  many  ran  away, 
damaging  the  vehicles  and  injuring  the  occupants. 

Rain  fell  in  torrents  with  the  hail  (0.80  inch  falling  between  3.45  p.  m.  and 
4  p.  m.),  poured  through  the  shattered  skylights  and  windows,  and  flooded 
houses.  The  streets  were  like  rivers,  and  in  many  places  the  street-car  tracks 
were  covered  to  a  depth  of  6  inches  by  the  soil  washed  down  from  adjacent 
hills. 

To  add  to  the  general  disaster  the  wind  blew  with  great  violence,  unroofing 
buildings,  breaking  in  the  remains  of  windows,  uprooting  trees,  and  giving 
the  hail  stones  a  dangerous  slant  to  the  eastward.  For  15  minutes  the  city 
was  in  a  state  of  complete  panic,  and  then  the  storm  passed  away  almost  as 
quickly  as  it  had  come.  A  half-hour  after  the  storm  had  left  the  city,  the 
rain  had  nearly  ceased,  the  wind  was  again  light,  and  a  rainbow  appeared 
in  the  east. 

The  hail  stones  M'ere  as  large  as  hen's  eggs.  Several  measured  more  than 
2  inches  in  diameter.  Three  weighed  together  12  ounces.  Some  as  large 
as  a  man's  fist  were  reported  by  reliable  parties  as  having  fallen  in  West 
Baltimore,  where  the  damage  by  hail  was  greatest.  The  hail  stones  were  of 
various  formations.  One  large  stone  was  covered,  on  the  side  unbroken  by 
its  fall,  with  a  number  of  sharp-pointed  prisms,  and  there  were  many  others 
like  it.  A  large  number  were  oval  in  form,  and  these  on  examination,  ex- 
hibited a  lamellated  structure,  being  composed  of  alternate  layers  of  trans- 
parent and  opaque  ice,  commencing  at  the  center  with  an  opaque  nucleus. 
Others  were  spheroidal  in  form  and  were  similar  in  structure  to  the  oval 
ones,  and  like  them,  very  hard.  The  large  prism-covered  stones  were  com- 
posed, in  the  center,  of  a  mass  of  sponge  ice,  and  were  generally  crushed  upon 
striking  the  pavement;  no  lamellated  structure  was  observed  in  these. 

Although  the  rain  commenced  falling  at  3.40  p.  m.  and  ended  at  5  p.  m., 
it  was  excessive  only  between  3.45  p.  m.  and  4  p.  m.  when  the  0.80  inch 
referred  to  above  fell. 

There  was  a  marked  fall  in  temperature  during  the  progress  of  the  storm. 
The  maximum  of  the  day,  71°,  occurred  some  time  before  the  storm's  arrival. 
Between  3.30  p.  m.  and  4  p.  m.  the  temperature  fell  from  67°  to  52°,  rising 
again  to  60°  after  the  storm  had  passed. 

The  wind,  which  had  been  very  light  all  day,  upon  the  approach  of  the 
storm  suddenly  veered  from  the  southeast  to  west-northwest  and  blew  with 
rapidly  increasing  force,  reaching  a  velocity  of  30  miles  per  hour  at  5  minutes 
before  4  o'clock.     After  4  o'clock  it  decreased  in  force  and  again  became  light. 

From  information  received  from  suburban  points,  it  is  estimated  that  the 
hail  band  began  about  10  miles  west-northwest  of  the  city  and  terminated 
5  miles  east  of  the  city,  and  extended  in  breadth  from  the  southern  limits  to 


MARYLAND   WEATHER   SERVICE  421 

5  miles  to  the  north.     This  would  make  the  baud  about  25  miles  long  and 
about  10  miles  broad. 

The  amount  of  property  destroyed  is  estimated  at  from  $60,000  to  $100,000, 
the  damage  for  the  most  part  being  confined  to  windows  of  western  ex- 
posure— a  great  many  thousands  of  which  were  broken — and  to  skylights 
and  greenhouses.  A  few  windows  with  a  northern  exposure  were  also 
broken.  The  damage  from  the  wind  was  great,  a  number  of  houses  losing 
their  roofs,  while  many  trees  in  all  parts  of  the  city  were  blown  down. 
There  was  no  loss  of  life. 

The  physical  structure  of  hail  stones  is  well  known.  There  is  a 
central  nucleus  of  opaque  snow  or  ice,  consisting  of  snowflakes  and  ice 
crystals  mixed  with  air  bubbles.  This  central  nucleus  is  surrounded 
by  a  series  of  thin  alternating  layers  of  clear  ice  and  opaque  snowy 
material.  There  may  be  as  many  as  10  or  12  of  these  layers.  The  diam- 
eters of  the  stones  vary  from  a  few  tenths  of  an  inch  to  three  and  even 
four  inches.     The  variation  in  the  shape  of  the  stones  is  also  very  great. 

Although  the  theory  of  hail  formation  has  received  a  great  deal  of 
attention  it  is  still  in  a  very  unsatisfactory  state.  The  explanation  of 
the  method  of  formation  of  the  successive  layers  of  packed  snow  and 
clear  ice  presents  great  difficulties  as  we  are  obliged  to  rely  almost  wholly 
upon  speculation  as  to  the  physical  processes  which  go  on  within  the 
heavy  cloud  mass  which  constitutes  the  laboratory  of  the  hail  stone.  The 
outward  form  of  the  tall  '"  thunder  head  "  identified  with  hail  storms  is 
being  carefully  studied,  and  we  may  hope  soon  by  means  of  instruments 
carried  aloft  by  kites  and  balloons  to  gather  some  valuable  facts  as  to  the 
physical  processes  going  on  within,  w^hich  will  eventually  lead  to  a  better 
understanding  of  hail  formation. 

Spring  Frosts. 

Marked  falls  in  temperature  have  a  special  significance  in  April  and 
the  early  part  of  May  in  most  sections  of  the  Middle  Atlantic  states. 
During  an  average  season  spring  fruits  have  passed  the  critical  period 
of  injurious  frosts  by  the  middle  of  April  in  the  vicinity  of  Baltimore, 
as  the  average  occurrence  of  the  last  killing  frost  falls  within  the  first 
week  of  this  month.  Frequently,  however,  a  killing  frost  will  occur  in  the 
latter  part  of  April,  and  on  rare  occasions  in  the  first  decade  of  May. 


422 


THE    CLIMATE    OF   BALTIMORE 


During  April  light  to  heavy  frosts  are  generally  looked  for  when  a 
pronounced  area  of  high  pressure  (an  anti-cyclone)  passes  directly  over 
the  Middle  Atlantic  states  from  the  west  or  northwest.  In  addition  to 
the  fall  in  temperature  occasioned  by  the  actual  transfer  of  masses  of  cold 
air  from  the  northwest  or  north  into  the  Middle  Atlantic  states,  there  is 
a  still  further  reduction  in  temperature,  owing  to  the  rapid  loss  of  heat 


Fig.  149.— The  Frost  of  May  9,  1906. 


during  the  night  within  the  anti-cyclone ;  the  dry  air  and  cloudless  skies 
accompanying  these  areas  facilitating  rapid  radiation  from  the  ground. 
When,  as  frequently  happens,  the  anti-cyclone  is  preceded  by  a  baro- 
metric depression  accompanied  by  rain,  the  probability  of  the  occurrence 
of  frost  is  greatly  increased,  as  the  atmosphere  is  then  charged  with 
moisture  on  the  approach  of  the  fall  in  temperature  within  the  anti- 
cyclonic  area.  If  at  the  time  of  the  8  p.  m.  observation  the  temperature 
is  between  40°  and  50°  at  Baltimore,  and  the  arrival  of  a  pronounced 


ilAEYLAND   WEATHER   SERVICE  423 

anti-cjclone  is  expected  during  the  following  night  from  the  west  or 
northwest,  frosts  are  very  likely  to  occur  in  the  early  morning  hours. 
The  injury  resulting  from  a  frost  depends,  not  only  on  the  fall  in  tempera- 
ture, but  also  upon  the  state  of  vegetation,  the  amount  of  moisture  in  the 
atmosphere,  and  upon  the  wind  movement.  Clear,  quiet  nights  greatly 
facilitate  the  production  of  frost  in  the  lower  places,  allowing  the  colder, 
heavier  air  to  settle  near  the  ground.  A  clouded  sky  will  prevent  rapid 
radiation  from  the  ground.  A  moderate  -wind  movement,  by  thoroughly 
mixing  the  air,  will  prevent  any  great  difference  in  temperature  between 
the  layers  near  the  ground  and  the  air  at  higher  levels.  A  frost  occurring 
shortly  after  the  appearance  of  tender  plants  is  likely  to  do  more  damage 
than  a  heavier  frost  later  on  when  the  plant  has  become  more  vigorous. 

For  details  concerning  the  occurrence  of  spring  frosts  see  pages  129 
to  135. 

The  general  weather  conditions  on  the  morning  of  May  9,  1906,  show 
a  situation  from  which  frost  may  be  expected  in  the  Middle  Atlantic 
states  during  the  following  night. 

Temperatures  ranging  between  28°  and  35°  occurred  in  nearly  all 
counties  of  Maryland  on  the  10th  and  11th.  This  anti-cyclone  was  the 
occasion  of  one  of  the  severest  spells  of  cold  weather  experienced  in 
Maryland  so  late  in  the  season.  All  stations  in  the  mountainous  portion 
of  the  state,  and  many  stations  elsewhere,  experienced  temperatures 
below  freezing.  Even  the  southern  portion  of  the  Eastern  Shore  Avas  not 
exempt,  Salisbury  reporting  29°  and  Princess  Anne  31°. 

At  a  number  of  stations  the  temperature  did  not  fall  below  40°.  The 
minimum  in  Baltimore  was  38°  on  the  10th.  While  the  frost  was  quite 
severe,  fruits  and  vegetables  were  too  far  advanced  to  suffer  any  very 
great  amount  of  injury.     (See  Fig.  149.) 

ICE    WITHOUT    FROST. 

The  weather  map  of  8  a.  m.,  April  17,  1905,  shows  freezing  conditions 
throughout  Maryland,  but  no  frosts  were  reported.  A  well  developed 
high  area  with  its  crest  extending  from  Montana  southeastward  to 
Florida  was  associated  with  a  deep  cyclone  centered  over  the  Lower  St. 

28 


424 


THE    CLIMATE    OF   BALTIMORE 


Lawrence  V'alley.  Strong,  steady  west  winds  prevailed  as  a  result  of  this 
distribution  of  pressure  over  the  Middle  Atlantic  states.  The  dry  air 
aided  by  considerable  movement  prevented  the  formation  of  frost, 
though  ice  formed  in  a  number  of  places.     (See  Fig.  150.) 

Periods  of  Unsettled  Weather. 
In  the  eastward  drift  of  cyclones  in  our  latitudes,  the  rain  area  occupies 
from  one  to  two  days  in  passing  a  given  point.     In  76  per  cent  of  all 


Fig.  150. — Ice  without  Frost,  April  17,  1905. 

occurrences  of  precipitation  the  rain  or  snow  falls  within  a  forty-eight 
hour  limit  in  Baltimore.  In  13  per  cent  of  instances  the  precipitation 
covers  all  or  a  portion  of  three  days.  This  leaves  very  little  margin  for 
extended  periods  of  consecutive  days  with  rain  or  snow.  (See  page  213.) 
Long  periods  of  unsettled  weather  with  rain  or  snow  are  of  most  frequent 
occurrence  in  the  spring  season. 


MARYLAND   WEATHER   SERVICE  425 

PERIODS    OF    UNSETTLED    WEATHER. 

(With  6  or  more  consecutive  days  of  rain  or  snow.) 

D.    J.       F.      M.    A.   M.      J.      J.    A.      S.      O.    N.  Y. 

Total  frequency  ill  34  years 13      II      15     20      14      23      13      12      IT       9       7      11  161 

Maximum  duration  (days) 22     19     IT      22     13     23      18      15      19      12      10      10  23 

Number  of  intervening  daj'S  with- 
out rain 644214324001  4 

Seasonal  frequency 38                      57                      42                      27  164 

These  periods  of  unsettled  weather  may  be  due  to  a  great  variety  of 
causes.  There  is  not  a  regular  and  periodic  succession  of  well  developed 
cyclones  and  anti-cyclones,  such  as  have  been  described  in  preceding 
pages.  The  well  developed  types  have  been  selected  for  illustrative  pur- 
poses, as  they  are  simple  in  outline  and  more  readily  interpreted.  In 
studying  the  actual  weather  map  from  day  to  day  we  find  there  are  no 
two  weather  conditions  exactly  alike ;  there  is  an  infinite  variety  in  the 
outline  of  isobars,  the  trend  of  isotherms,  the  shape  of  rain,  areas,  etc. 
An  unusual  succession  of  rainy  days  may  be  due  to  the  very  slow  move- 
ment of  a  cyclonic  area,  to  a  rapid  succession  of  storms,  to  a  recurving 
of  the  storm  upon  its  path,  or  to  a  combination  of  these  causes.  It  may 
also  be  due  to  the  persistence  of  a  so-called  "  flat  map,"  a  map  without 
well  defined  cyclones  or  anti-cyclones. 

THE  RAINY  PERIOD  OF  APRIL  19-25,  1901. 

On  the  morning  of  April  16,  1901,  a  depression  entered  the  United 
States  in  the  extreme  Southwest;  at  8  a.  m.  its  center  was  over  Arizona. 
This  depression  travelled  slowly  eastward,  causing  moderate  to  heavy 
rains  over  Texas  and  the  West  Gulf  states  during  the  ITth  and  18th. 
By  the  morning  of  the  18th  the  center  was  over  Alabama.  In  connec- 
tion with  another  depression  centered  over  Ohio  a  long  trougli  of  low 
pressure  was  formed  extending  from  the  Lower  Lake  region  to  the  Gulf 
of  Mexico.  By  the  morning  of  the  19th  the  two  depressions  had  merged 
into  a  single  storm  with  its  center  over  Georgia.  Under  the  influence  of 
these  two  storm  centers,  aided  by  an  area  of  high  pressure  in  the  extreme 
Northeast,  east  to  northeast  winds  set  in  at  Baltimore,  and  rain  began  to 
fall  on  tlie  19th.  The  storm  turned  sharply  up  the  coast  on  the  19th,  ac- 
companied by  a  very  heavy  rainfall.     At  8  a.  m.  of  the  20th  the  center  was 


426  THE    CLIMATE    OF   BALTIMORE 

over  North  Carolina  and  Southern  Virginia.  From  the  morning  of  the 
20th  to  the  evening  of  the  21st  the  storm  center  had  travelled  only  from 
Southern  Virginia  to  Western  Maryland.  On  the  morning  of  the  22d  the 
center  was  found  over  the  Ohio  Valley,  having  been  deflected  westward — a 
rare  occurrence.  The  winds  at  Baltimore  continued  to  blow  from  an  east- 
erly direction.  This  depression  began  to  fill  up  and  two  secondary  centers 
of  low  pressure  developed  along  the  coast,  one  over  Eastern  Maryland  and 
the  other  over  the  South  Atlantic  states.  By  the  morning  of  the  24th 
these  two  secondary  depressions  had  merged  into  a  single  storm  center 
off  the  coast  of  Delaware  and  New  Jersey.  This  storm  moved  slowly 
northeastward  just  off  the  coast,  disappearing  by  the  morning  of  tho 
26th. 

This  very  slow  movement  and  peculiar  path  of  the  original  storm, 
and  the  subsequent  formation  and  sluggish  progress  of  the  secondary 
depressions  in  the  neighborhood  of  Maryland  kept  Baltimore  within 
their  rain  areas  for  six  successive  days.  The  amounts  recorded  upon 
each  day  of  this  period  were  not  large,  but  the  six  days'  total  exceeded 
two  inches. 

DAILY  RAINFALL  APRIL  19  TO  25,  1901. 

April  19,  1901 0.06  inch. 

20,  "     0.56 

21,  "     0.54       " 

22,  "     Trace 

23,  "     0.11       " 

24,  "     0.71       " 

25,  "      0.05 

Total    2.03    inches. 

The  rainfall  period  lasted  162  hours,  but  the  precipitation  was  not 
continuous,  scattered  showers  occurring  on  the  21st,  22d,  23d,  and  25th. 

THE  RAINY  PERIOD  OF  MAY  lG-26,  1894. 

There  is  a  general  impression  that  rainy  periods  of  much  greater 
length  than  that  recorded  above  are  of  frequent  occurrence;  a  close 
inspection  of  records,  however,  will  reveal  the  fact  that  there  are  inter- 
vening days  without  a  trace  of  rain. 


MARYLAND   WEATHER   SERVICE  427 

One  of  the  longest  periods  noted  in  the  Baltimore  records  of  unsettled 
weather  with  daily  rainfall  was  that  of  the  16th  to  the  26th  of  May, 
1894. 

This  period  was  connected  with  the  passage  of  a  Lake  storm  of  great 
extent  and  energy.  Here  again  the  path  of  the  storm  after  leaving  the 
Lake  region  was  peculiar,  while  the  progress  was  very  slow — in  fact 
the  center  was  nearly  stationary  in  the  vicinity  of  Maryland  for  the 
greater  part  of  three  days.  The  storm  had  its  origin  over  the  Northern 
Eocky  Mountain  slope  on  the  15th.  It  moved  slowly  to  the  Lower 
Lake  region  until  the  18th,  then  dipped  abruptly  southward  and  remained 
over  Virginia,  Maryland,  and  West  Virginia  until  the  depression  gradually 
filled  up  on  the  21st.  In  the  meantime  a  second  depression  developed  over 
Georgia  and  Xorth  Carolina  and  moved  slowly  up  the  coast,  disappearing 
off  the  coast  of  New  England  on  the  26th.  From  the  16th  to  the  26th 
Baltimore  was  within  the  rain  areas  of  these  two  storms  and  much  of  the 
time  very  near  their  centers. 

The  rainfall  recorded  during  this  period  was  as  follows: 

DAILY   RAINFALL  AT   BALTIMORE   FROM   MAY   16-26,    1894. 
May  16,  1894 0.13    inch  May  22,  1894 0.01    inch 


17,  "  0.16 

18,  "  0.51 

19,  "  0.09 

20,  "  1.07 

21,  "  0.19 


23,  "     1.34 

24,  "     0.16 

25,  "     0.05 

26,  "     0.14 

Total 4.45 


While  the  entire  period  covered  by  the  rainfall  was  a  little  over  10 
days,  there  were  but  63  hours  of  actual  rainfall.  (See  pages  174  and 
219  et  seq.  for  frequency  and  duration  of  wet  spells.) 

The  most  notable  instances  of  a  long  continued  rainfall  occurring 
since  hourly  records  were  begun  by  the  United  States  Weather  Bureau  in 
1893  at  Baltimore,  was  that  of  April  27  to  May  1,  1895.  The  records 
show  that  rain  fell  for  102  consecutive  hours.  Though  the  rain  was 
reduced  to  a  light  mist  at  times,  it  never  entirely  ceased  during  this 
period.     The  total  amount  of  rainfall  was  3.69  inches.     There  was  no 


428  THE    CLIMATE    OF   BALTIMORE 

well  defined  storm  area  in  the  vicinity  of  Baltimore.     The  barometer 

was  high  over  the  New  England  states,  and  a  shallow  though  ill  defined 

depression  covered  the  Gulf  of  Mexico;  the  depression  moved  slowly 

northward  and  eastward  some  distance  off  the  coast,  causing  a  steady 

northeast  wind  at  Baltimore — a  mild  "  northeaster." 

There  is  a  myth  associated  with  the  occurrence  of  rainfall  on  St. 

Swithin's  day  which  seldom  fails  to  receive  the  attention  of  the  press  on 

the  15th  of  July: 

"  St.  Swithin's  Day,  if  ye  do  rain. 
For  forty  days  it  will  remain; 
St.  Swithin's  Day,  an  ye  be  fair, 
For  forty  days  'twill  rain  nae  mair." 

The  nearest  approach  to  a  fulfillment  of  this  prophecy,  according  to 
the  Baltimore  rainfall  records  of  the  past  36  years,  is  a  period  of  9  con- 
secutive days  with  rain  in  the  month  of  July.     (See  page  213.) 

The  Variability  or  Weather  in  Spring. 

As  an  illustration  of  the  changeableness  of  weather  conditions  in  the 
spring  of  the  3'ear,  we  may  present  the  irregular  fluctuations  from  day 
to  day,  or  we  may  show  the  changes  which  have  taken  place  upon  the 
same  calendar  day  of  each  year  for  a  long  series  of  years.  As  regards 
the  degi'ee  of  variability  in  temperature  the  month  of  March  ranks  with 
the  winter  months.  Throughout  the  early  spring  rapid  fluctuations  and 
strong  contrasts  in  the  conditions  of  successive  days  are  of  common 
occurrence. 

Owing  to  the  general  interest  in  the  character  of  the  weather  on  the 
4th  of  March,  at  least  once  in  four  years,  this  day  is  selected  as  a  type 
of  March  weather.  Whatever  reason  there  may  be  from  a  historical 
point  of  view  in  favor  of  continuing  to  inaugurate  our  presidents  on  the 
fourth  of  March  there  is  little  to  recommend  the  day  in  past  experience 
of  weather  conditions  and  in  the  small  chances  for  a  favorable  day.  The 
day  falls  within  a  period  of  rapid  warming  up  in  the  northern  hemi- 
sphere, but  the  advancing  sun  has  not  yet  carried  the  day  beyond  the 
realm  of  freezing  weather.  The  early  morning  temperatures  are  nearly 
always  below  32°,  while  the  frequent  incursions  of  cold  air  from  the 


MARYLAND   WEATHER   SERVICE  429 

north  and  west  bring  even  the  average  heat  of  the  day  close  to  the  freez- 
ing point  on  most  occasions. 

Added  to  the  discomforts  of  a  raw  cold  atmosphere  we  have  the  pro- 
verbial March  wiads,  not  infrequently  combined  with  sleet  or  rain  or 
snow,  or  a  combination  of  all  of  these  disagreeable  elements. 

The  probability  for  a  fine  day  is  so  small  that  it  is  surprising  that  the 
efforts  to  change  the  date  of  inauguration  to  the  latter  part  of  the  fol- 
lowing month  have  not  yet  been  successful. 

THE  WEATHER  OF  MARCH  4. 

The  condition  of  the  weather  on  the  4th  of  March  is  graphically  repre- 
sented in  the  accompanying  diagram  for  each  year  since  1871.  It  might 
have  added  interest  to  carry  the  diagram  back  to  an  earlier  date  but  the 
period  of  37  years  represents  practically  all  the  chief  combinations  likely 
to  have  occurred  in  the  longer  period.  While  the  conditions  charted 
represent  Baltimore  weather,  the  close  proximity  of  Washington  makes 
it  improbable  that  there  were  at  any  time  any  material  differences  in 
the  weather  conditions  of  the  two  cities.  There  is  most  certainly  no 
difference  in  the  variability  of  the  elements. 

The  average  daily  temperature  on  the  4th  of  March  is  not  far  from  the 
point  of  frost  formation.  With  the  usual  daily  range  the  fluctuations 
will  be  above  and  below  the  frost  line.  When  precipitation  takes  place 
it  is  apt  to  change  from  rain  to  snow  or  from  snow  to  rain,  with  the 
intermediate  stage  of  sleet. 

In  1873  the  temperature  fell  to  5°  above  zero  in  the  early  morning  of 
the  4th;  on  the  following  4th  of  March,  1874,  the  afternoon  temperature 
registered  68°  above.  In  1880  the  temperature  rose  as  high  as  74°. 
Between  these  widely  separated  limits  the  temperature  has  kept  up  a 
continual  see-saw  about  the  middle  point  of  38°. 

In  the  past  37  years  rain,  snow,  or  sleet  has  fallen  on  16  occasions, 
just  46  per  cent  of  the  days.  The  average  amount  of  precipitation  is 
about  half  an  inch  and  the  average  duration  about  9  hours.  The  sky 
has  been  overcast  10  times,  partly  clouded  15  times,  and  clear  11  times. 
The  prevailing  winds  have  been  from  the  west  and  northwest. 


o      o      o     o 

!>.          to         cO        t- 

^    1 
o     ^ 

i.?  i  - 

C 

>       ^ 

- 

5          O          lO 

-      —      do 

^   i 

; 

^rh' 

■  '-^      ! 

I 
'- 1 

i 

/. 

^     ^ 

u 

ii:; 

X.:  n  i 

; 

I 

o 

'ii 

4  ^ 

/ 

r; 

■^  ^  ^  :  > 

; 

1 

£ 

A 

v\ 

/  ^ 

N 

v- 

X          : 

1 

f ; 

UJ 

or 

\i 

i 

s 
^ 

j 

^: 

/I 

/ 

or.     J^^   : 

-5 

CC 

< 

1 

ai' 

V 

\  \ 

\ 

o 

t9i 

a> 

y 

^' 

\s 

^IV;^ 

en 

i 

V 

\  : 

X^ 

/>; 

^A- 

/  : 

I 

S«- 

<;;. 

^S 

\ 

^i'' 

i 

it^ 

-<'■' 

>y 

:^^' 

,•*  : 

N 

sN, 

\ 

Ai' 

11  '■ 

© 

// 

i  V 

:   ^ 

^ 

i/S;  \ 

i 

i 

^ 

\ 

'  ^ 

H 

i9i 
irTi'  N 

\ 

/ 

i^^ 

^  ■ 

}  / 

■rih' ^ 

i 

/ 1 

1 

Ia^ 

o 

/ 

k/' 

J 

:0  V 

1 

£ 

i 

<;■- 

"^  ^ 

•n^ ' 

f  ■ 

■ 

>/^ 

/ 

^n'^ 

^  ■ 

; 

X 

]/'  ; 

^S^' 

;»■  • 

V 

sX,  \ 

^     ■ 

^ 

•2^ ' 

! 

? 

.-^ 

^ 

=^'' 

5^ 

/ 

'  ^rh^  ■ 

i 

*^ 

^ 

^> 

>^^ 

/ 

1 

|Ti^ 

! 

f 

// 

V 

N.^ 

jT: 

1>  '■ 

LlJ 

ct- 

a. 

5 

Zl     5 

(  : 

;9;' 

fl  : 

i 

s 

s;- 

s 

;Y/ 

/  ^ 

;      1 

^     ^ 

o 

o  r^  ■ 

2S 

^ 

^y 

^"i 

i 

1^ 

T^ 

^^ 

? 

LlJ    j^  : 

s 

«  ^ 

i  .•-' 

^^ 

/ 

s 

iX^ 

/ 

:i^ 

/ 

:0:  ) 

:  I/O      ■ 

r 

X         mm 

-     ! 

i 

i        ;V;    1 

:  2 

^      1 

N         —         — 


MARYLAND   WEATHER   SERVICE  431 

THE  WEATHER   OF   MARCH  4. 


Year. 

Max. 

Min. 

Mean 

Character 

Wind 

Daily  Wind 

Precip- 

Temp. 

Temp. 

Temp. 

of  Day. 

Direction. 

Movement. 

itation. 

(De 

grees  Fah 

r.) 

(Miles) 

(Inches) 

1871 

44 

39 

41 

Pt.  cldy 

N 

0.51 

1872 

44 

IS 

31 

Clear 

SW  &  NW 

106 

1873 

21 

5 

13 

Pt.  cldy 

NW 

362 

1874 

68 

42 

55 

Pt.  cldy 

NW 

188 

0.21 

1875 

35 

27 

31 

Clear 

NW 

197 

1876 

42 

27 

34 

Pt.  cldy 

SE 

138 

1877 

57 

33 

45 

Pt.  cldy 

NW 

137 

0.09 

1878 

53 

35 

44 

Pt.  cldy 

NW 

206 

1879 

45 

26 

36 

Cloudy 

NE 

76 

0.02 

1880 

74 

48 

62 

Pt.  cldy 

W 

223 

0.01 

1881 

38 

32 

35 

Pt.  cldy 

w 

331 

1.13 

1882 

57 

40 

48 

Clear 

NW 

141 

1883 

43 

28 

36 

Pt.  cldy 

NW 

218 

1884 

32 

18 

25 

Clear 

NW 

214 

1885 

53 

24 

44 

Cloudy 

E 

95 

1886 

42 

29 

36 

Pt.  cldy 

NW 

236 

1887 

36 

27 

32 

Cloudy 

NE 

117 

0.45 

1888 

35 

24 

30 

Clear 

NW 

198 

1889 

44 

36 

40 

Cloudy 

NE&NW 

309 

2.71 

1890 

48 

27 

38 

Clear 

NE 

133 

1891 

42 

30 

36 

Pt.  cldy 

NW 

206 

6.18 

1892 

55 

35 

45 

Cloudy 

NW 

181 

1893 

31 

24 

28 

Pt.  cldy 

NW 

438 

0.18 

1894 

56 

31 

44 

Clear 

SE 

110 

1895 

55 

36 

46 

Pt.  cldy 

SW 

306 

0.02 

1896 

42 

23 

32 

Clear 

N 

458 

1897 

47 

35 

41 

Clear 

E 

117 

1898 

40 

35 

38 

Cloudy 

N 

205 

0.13 

1899 

44 

36 

40 

Cloudy 

E 

171 

0.07 

1900 

53 

31 

42 

Cloudy 

SW 

65 

1901 

50 

35 

42 

Cloudy 

NW 

128 

0.21 

1902 

43 

32 

38 

Cloudy 

W 

108 

0.03 

1903 

54 

35 

44 

Pt.  cldy 

SE 

84 

1904 

35 

24 

30 

Clear 

NW 

244 

1905 

47 

27 

37 

Pt.  cldy 

N 

250 

0.01 

1906 

54 

38 

46 

Pt.  cldy 

NW 

340 

1907 

39 

24 

32 

Clear 

NW 

185 

Means 

45 

31 

38 

Pt.  cldy 

175 

0.53 

432  THE    CLIMATE    OF   BALTIMORE 

THE  WEATHER  OF  MAY  1. 

The  closing  days  of  April  and  the  first  days  of  May  are  among  the 
pleasantest  of  the  year  in  many  respects.  The  temperature  is  well  above 
the  frost  line — the  mean  temperature  for  the  first  of  May  is  59°.  The 
early  morning  temperatures  are  more  constant  than  in  March  or  April, 
the  minimum  averaging  about  50°.  The  maximum  has  gone  as  high  as 
85°,  but  it  has  generally  been  below  70°.  The  winds  are  light  and 
largely  from  a  southerly  or  easterly  direction.     (See  Fig.  152.) 

The  rainy  days  are  less  frequent  than  earlier  in  the  season;  there  are 
few  days  in  the  year  with  a  smaller  rainfall  probability  (see  Plate  IX). 
The  duration  of  rainfall  is  about  7  hours  as  compared  with  9  hours  in 
March  and  10  to  12  hours  in  the  winter  months.  The  average  amount 
of  rainfall  is  also  quite  small  compared  with  that  of  other  days. 

The  chances  for  fine  weather — a  moderate  temperature  and  without 
rain — are  better  for  the  30th  of  April  and  the  1st  of  May  than  for  any 
period  of  the  year,  excepting  the  first  week  in  September  and  the  middle 
of  October. 

The  weather  conditions  on  the  4th  of  March  and  on  the  first  of  May 
represent  with  a  fair  degree  of  accuracy  the  conditions  of  the  early  and 
late  spring  respectively.  They  represent  sharp  contrasts — the  former 
showing  all  the  characteristics  of  our  variable  winter  climate,  while  the 
latter  resembles  closely  the  more  uniform  and  settled  conditions  of  our 
summers. 

THE  WEATHER  OF  EASTER  SUNDAY. 

The  weather  of  this  day  has  been  prevailingly  cloudy  to  partly  cloudy, 
with  a  moderate  westerly  wind.  Eain  has  occurred  on  14  of  the  37 
anniversaries  from  1871  to  1907,  and  was  light  in  amount  on  all  but 
one  occasion.  With  an  average  temperature  of  52°,  the  extremes  have 
ranged  between  84°  in  1887  and  28°  in  1874. 


434 


THE    CLIMATE    OF   BALTIMORE 


THE  WEATHER  OF  MAY  1. 


Year. 

1871 

Max.           Min. 
Temp.       Temp. 
(Degrees  Fahr.) 
67              58 

Mean 
Temp. 

62 

Character 
of  Day. 

Cloudy 

Wind 
Direction. 

E 

Daily  AVind 
Movement. 

(Miles) 

Precip- 
itation. 
(Inches) 

1872 

73 

59 

66 

Cloudy 

SE 

175 

1873 

59 

47 

53 

Cloudy 

S 

127 

0.24 

1874 

70 

47 

58 

Cloudy 

W 

184 

1875 

61 

47 

54 

Cloudy 

E 

214 

0.01 

1876 

61 

34 

48 

Clear 

NW 

324 

1877 

53 

44 

48 

Cloudy 

NW 

109 

1878 

80 

52 

66 

Pt.  cldy 

SB-SW-W 

82 

1879 

65 

45 

55 

Pt.  cldy 

NW 

249 

1880 

61 

38 

50 

Clear 

NW 

245 

1881 

67 

46 

56 

Clear 

SE 

175 

1882 

68 

47 

58 

Clear 

W&NW 

131 

1883 

60 

45 

52 

Pt.  cldy 

SE 

125 

1884 

74 

57 

66 

Pt.  cldy 

SE&S 

155 

1885 

62 

48 

55 

Cloudy 

NE 

97 

0.23 

1886 

58 

46 

52 

Cloudy 

NE 

163 

1887 

70 

52 

61 

Pt.  cldy 

SE 

110 

1888 

76 

54 

65 

Pt.  cldy 

NW 

151 

0.04 

1889 

54 

48 

51 

Pt.  cldy 

N&SW 

100 

0.23 

1890 

87 

58 

72 

Pt.  cldy 

S&NE 

144 

0.30 

1891 

80 

64 

72 

Clear 

W 

161 

1892 

77 

49 

63 

Clear 

SE&S 

293 

1893 

64   , 

49 

56 

Cloudy 

E 

194 

0.02 

1894 

80 

50 

65 

Clear 

SW 

124 

1895 

57 

54 

56 

Pt.  cldy 

NE 

353 

0.12 

1896 

54 

50 

52 

Cloudy 

SE 

225 

1897 

69 

57 

63 

Cloudy 

E 

287 

0.94 

1898 

79 

50 

64 

Pt.  cldy 

SE&NW 

73 

1899 

84 

58 

71 

Clear 

SE 

101 

1900 

77     . 

53 

65 

Pt.  cldy 

W 

81 

... 

1901 

78 

51 

64 

Pt.  cldy 

E 

171 

1902 

77 

56 

66 

Clear 

NW 

187 

1903 

69 

44 

56 

Clear 

NW 

306 

1904 

72 

49 

60 

Pt.  cldy 

NW 

136 

1905 

62 

48 

55 

Clear 

NW 

198 

1906 

75 

59 

67 

Cloudy 

N 

140 

... 

1907 
Means 

63 
68 

52 
50 

58 
59 

Cloudy 
Pt.  cldy 

N 

189 
174 

0.04 
0.22 

MARYLAND   WEATHER   SERVICE 


435 


THE   WEATHER  OF  EASTER  SUNDAYS. 


Year. 

Date. 

Max. 
Temp. 

Min. 
Temp. 

Mean 
Temp. 

Character 
ot  Day. 

Wind 
Direc- 
tion. 

Daily      Pre- 
Wind    cipita- 
Movement.  tion. 

(Degrees  Fahr.) 

(Miles)  (Inches) 

1871 

April 

9 

82 

73 

78 

Pt.  cldy 

W 

1872 

March 

31 

64 

44 

54 

Cloudy 

W 

183 

0.32 

1873 

April 

13 

60 

42 

51 

Pt.  cldy 

NW 

244 

0.01 

1874 

April 

5 

42 

28 

35 

Cloudy 

SE 

174 

0.10 

1875 

March 

28 

54 

35 

44 

Pt.  cldy 

S 

137 

1876 

April 

16 

65 

49 

57 

Cloudy 

SW 

212 

1877 

April 

1 

59 

42 

50 

Cloudy 

SE 

116 

0.02 

1878 

April 

21 

79 

60 

70 

Clear 

NW 

156 

1879 

April 

13 

58 

35 

46 

Pt.  cldy 

SE 

157 

1880 

March 

28 

59 

38 

48 

Cloudy 

NW 

99 

0.06 

1881 

April 

17 

64 

44 

54 

Pt.  cldy 

W 

227 

1882 

April 

9 

56 

49 

53 

Cloudy 

S 

97 

0.38 

1883 

March 

25 

46 

30 

38 

Cloudy 

SE 

22 

1884 

April 

13 

55 

47 

51 

Pt.  cldy 

SW 

89 

0.15 

1885 

April 

5 

60 

35 

48 

Pt.  cldy 

SW 

241 

1886 

April 

25 

80 

54 

67 

Pt.  cldy 

NE 

142 

0.01 

1887 

April 

10 

84 

46 

Go 

Clear 

NW 

102 

1888 

April 

1 

58 

43 

50 

Pt.  cldy 

SE 

160 

1889 

April 

21 

80 

60 

70 

Clear 

NW 

172 

1890 

April 

6 

62 

36 

49 

Clear 

SW 

115 

1891 

March 

29 

50 

38 

44 

Clear 

N 

201 

1892 

April 

17 

58 

42 

50 

Pt.  cldy 

NE 

158 

1893 

April 

2 

62 

47 

54 

Clear 

NW 

266 

1894 

March 

25 

45 

39 

42 

Cloudy 

N&NW 

137 

O.Ol 

1895 

April 

14 

59 

43 

51 

Clear 

N 

178 

1896 

April 

5 

55 

33 

44 

Clear 

NW 

209 

1897 

April 

18 

61 

41 

51 

Pt.  cldy 

W 

103 

1898 

April 

10 

63 

47 

55 

Pt.  cldy 

E 

86 

0.01 

1899 

April 

2 

43 

31 

O  1 

Pt.  cldy 

W 

146 

1900 

April 

15 

65 

40 

52 

Clear 

SW 

75 

1901 

April 

7 

56 

46 

51 

Cloudy 

W 

154 

0.02 

1902 

March 

30 

62 

45 

54 

Pt.  cldy 

W 

118 

0.24 

1903 

April 

12 

55 

48 

52 

Cloudy 

E 

175 

1904 

April 

3 

46 

33 

40 

Cloudy 

NW 

295 

1905 

April 

23 

64 

45 

54 

Clear 

N 

172 

1906 

April 

15 

67 

51 

59 

Cloudy 

NW 

183 

1.07 

1907 

March 

31 

56 

42 

49 

Cloudy 

N 

168 

0.03 

Means 


60 


43 


52 


Pt.  cldy 


157 


0.24 


436  THE    CLIMATE    OF   BALTIMORE 

SUMMER  WEATHER. 

As  the  spring  advances,  atmospheric  movements  on  a  large  scale  become 
more  sluggish.  Well  defined  cyclones  and  anti-cyclones  are  of  less  fre- 
quent occurrence  and  less  intense  in  their  development.  This  is  due, 
doubtless,  to  the  decreasing  contrasts  in  temperature  between  north  and 
south,  and  between  the  oceans  and  the  continents.  Attention  has  already 
been  called  to  the  relatively  great  differences  in  temperature  between 
Florida,  for  instance,  and  Montana,  in  the  winter  months,  compared 
with  the  differences  in  the  summer  months,  an  average  difference  of 
about  75°  in  January  increasing  to  100°  at  times,  as  compared  with 
about  30°  in  July. 

With  the  northward  movement  of  the  sun  the  whole  atmosphere  of 
the  northern  hemisphere  rapidly  rises  in  temperature  during  the  day. 
At  the  same  time  the  days  become  longer,  and  the  nights  shorter;  the 
loss  of  heat  during  the  long  winter  nights  over  the  continental  masses 
becomes  steadily  less. 

With  the  increasing  heat  of  the  summer  the  mass  of  the  air  over  the 
continents  becomes  specifically  lighter  than  that  over  the  oceans.  The 
general  surface  circulation  of  the  air  between  continents  and  oceans  is 
reversed.  In  the  winter  time  the  general  drift  at  the  surface  is  from 
continents  to  oceans,  in  the  summer  time  from  the  oceans  to  the  conti- 
nents. As  the  winter  area  of  high  pressure  over  the  northern-central 
portion  of  the  JSTorth  American  continent  diminishes  in  strength,  the 
Atlantic  high  area  increases  in  extent  and  intensity.  With  its  center 
usually  over  the  Azores,  it  extends  westward  across  the  Atlantic  Ocean 
to  the  South  Atlantic  states  in  the  summer  time.  With  this  change  in 
the  distribution  of  atmospheric  pressure  from  winter  to  summer  there 
is  a  change  in  the  prevailing  wind  direction.  Maryland,  in  common  with 
all  of  the  Middle  Atlantic  states,  has  a  prevailing  west  to  northwest  wind 
in  the" winter  months,  the  air  blowing  out  of  the  continental  high  area; 
in  the  summer  months  the  prevailing  direction  is  southeast  or  southwest, 
coming  from  the  Atlantic  high  area  to  the  southeast  of  the  Middle 
Atlantic  states. 


MARYLAND   WEATHER   SERVICE  437 

In  the  summer  season  the  paths  of  the  centers  of  cyclones  are  con- 
fined mostly  to  the  northern  tier  of  states,  the  Lake  region  and  the  St. 
Lawrence  Valley.  Hence  marked  cyclonic  changes  in  temperature  are 
infrequent  in  the  states  farther  south.  The  distinguishing  feature  of 
the  temperature  changes  is  the  diurnal  variation,  the  difference  between 
the  early  morning  and  the  afternoon  readings  of  the  thermometer.  In 
the  winter  and  early  spring  months  the  irregular  cyclonic  changes  are 
far  greater  than  the  diurnal  change.  This  conspicuous  prominence  of 
the  diurnal  fluctuation,  which  is  characteristic  of  tropical  climates,  is 
not  confined  to  temperature;  it  also  appears  in  the  wind  direction,  the 
rainfall  and  in  local  storm  frequency. 

The  increasing  magnitude  of  the  diurnal  period  in  the  summer  months 
at  Baltimore  has  already  been  discussed  in  considerable  detail  in  the 
first  part  of  the  report.  Further  attention  will  be  directed  to  character- 
istic weather  types  of  the  summer  season  in  the  following  pages,  especially 
to  conditions  which  give  rise  to  local  storms,  such  as  thunderstorms, 
squalls,  tornadoes,  and  to  hot  spells. 

Summer  Storms. 
A  glance  at  Fig.  77  and  Fig.  78,  on  page  217,  will  at  once  reveal  the 
existence  of  an  intimate  connection  between  high  temperatures  and  the 
occurrence  of  thunderstorms.  In  our  latitudes  these  turbulent  atmos- 
pheric disturbances  become  more  and  more  frequent  with  the  northward 
movement  of  the  sun,  increasing  steadily  from  December  to  July,  and 
then  rapidly  decreasing  to  December.  The  following  figures  show  the 
annual  average  frequency  for  tlie  vicinity  of  Baltimore : 

THUNDERSTORMS    RECORDED    AT    BALTIMORE    (187G-1904). 

Jan.      Feb.      Mar.     Apr.     Maj-.      June.     July.      Aug.      Sept.     Oct.      Nov.     Dec.    Year. 

13     11     20     33     107     156    17D     111     43     7     R     2     678 

Over  80  per  cent  of  the  total  annual  number  of  these  storms  occur  dur- 
ing the  hot  season — May  to  August — and  including  September  and  April, 
the  total  frequency  for  the  summer  half  year  amounts  to  92  per  cent, 
leaving  but  8  per  cent  for  the  winter  half  year.  The  same  intimate 
connection  with  change  in  temperature  is  shown  in  the  diurnal  period 
of  thunderstorm  occurrence.     (See  page  276.) 


438  THE    CLIMATE    OF   BALTIMORE 

HOURLY  FREQUENCY  OF  THUNDERSTORMS. 

Hoursonding 1        3       3       4        5       6        7        8       9        10       11        13 

A.  M 7        1        3        7        0        3        0        6        6  8  9        15 

P.  M 3;.'      46      76      78      61      65      69     38      34       31        16       13         Total  610 

More  than  two-thirds  of  all  thunderstorms  recorded  at  Baltimore  in 
28  years  began  during  the  seven  hours  from  noon  to  seven  p.  m.  Many 
of  the  storms  occurring  later  than  7  p.  m.  had  their  origin  in  the  early 
afternoon  hours  and  were  carried  eastward  several  hours  before  being 
dissipated. 

When  the  thunderstorm  does  occur  in  the  cold  season  it  is  in  connec- 
tion with  a  relatively  warm  inflow  of  air  towards  the  center  of  a  cyclone. 

During  the  summer  months  many  thunderstorms  occur  in  the  absence 
of  any  well  defined  general  cyclonic  depression.  During  a  period  of 
abnormally  high  temperatures  due  to  bright  sunshine  and  a  sluggish 
wind  circulation,  the  lower  layers  of  the  atmosphere  are  excessively 
heated,  resulting  in  a  marked  disturbance  in  the  normal  rate  of  decrease 
of  heat  with  elevation.  A  brisk  vertical  circulation  is  set  up,  which,  in 
the  presence  of  a  high  humidity,  results  in  rapid  cooling  of  the  ascending 
air,  and  formation  of  cumulus  clouds.  As  this  circulation  increases  in 
energy  the  cloud  soon  developes  into  a  "  thunder  head  "  with  its  accom- 
paniment of  heavy  rain,  lightning,  and  thunder.  Hence  dynamic  cool- 
ing of  warm  moist  convection  currents  is  the  chief  cause  of  thunder- 
storms of  the  summer  season.  When  these  storms  occur  in  connection 
with  a  general  cyclone  the  mass  of  warm  moist  air  which  produces  the 
thunder  cloud  is  mechanically  forced  upward  in  addition  to  rising  as  a 
convection  current.  The  thunderstorms  occurring  in  connection  with  a 
general  cyclone  are  likely  to  be  of  wider  extent  than  those  due  to  con- 
vection currents  alone,  and  the  cool  air  following  the  storm  is  apt  to  be 
more  lasting.  The  fall  in  temperature  following  the  local  heat  thunder- 
storm is  usually  of  brief  duration. 

THE  THUNDERSTORM  OF  JULY  20,  1902. 

On  July  20,  1902,  a  thunderstorm  of  unusual  severity  passed  over 
Baltimore.  Twelve  lives  were  lost,  while  several  hundred  houses  were  un- 
roofed or  otherwise  seriously  damaged,  involving  a  loss  of  over  $200,000. 


MARYLAND   WEATHER   SERVICE  439 

The  wind  attained  a  velocity  seldom  equalled  in  the  annals  of  Baltimore 
weather. 

On  tlie  morning  of  the  20th  the  weather  chart  of  the  United  States 
Weather  Bureau  showed  an  area  of  moderately  low  pressure  over  the 
Lower  Lake  region,  the  Middle  Atlantic  and  Southern  New  England 
states,  with  the  center  of  the  depression  over  Lake  Erie  and  Southern 
Michigan  at  8  a.  m.  Cloudy  and  rainy  weather  prevailed  over  a  wide 
area  about  the  center  of  the  oval  depression.  A  well  defined  area  of 
high  pressure  covered  the  country  between  the  Mississippi  Eiver  and  the 
Eocky  Mountains.  By  8  p.  m.  the  barometric  depression  had  moved 
eastward  with  its  center  over  Pennsylvania,  the  isobars  meanwhile  becom- 
ing nearly  circular.  The  weather  from  the  Mississippi  Eiver  to  the 
Atlantic  coast  and  from  the  Lake  region  to  the  Gulf  of  Mexico  was  in  an 
unsettled  condition,  thunderstorms  occurring  during  the  day  at  nearly 
every  reporting  station  of  the  Weather  Bureau  in  the  Middle  Atlantic, 
the  South  Atlantic  and  Gulf  states,  and  the  Lake  region,  accompanied 
in  most  cases  by  light  rains.  The  chart  for  8  p.  m.  shows  the  general 
weather  conditions  within  an  hour  or  two  after  the  occurrence  of  many 
of  the  local  storms  in  the  Middle  Atlantic  states.     (See  Fig.  153.) 

The  progressive  changes  in  the  elements  as  recorded  at  Baltimore 
during  the  day  were  particularly  interesting  and  instructive.  The  baro- 
meter slowly  and  steadily  fell  during  the  forenoon;  the  wind  blew  from 
the  southwest  with  a  velocity  of  only  4  to  8  miles  until  8  a.  m.,  then 
increased  steadily  to  15  or  16  miles  per  hour  by  10  a.  m.  The  tempera- 
ture rose  rapidly  from  a  minimum  of  72°  at  6  a.  m.  to  a  maximum  of 
94°  at  1  p.  m.  The  sky  was  overcast  during  the  night,  but  there  was 
considerable  sunshine  from  7  a.  m.  until  1  p.  m.,  when  a  sheet  of  stratus 
clouds  appeared  in  the  west,  intensely  dark  and  advancing  rapidly 
towards  the  zenith.  At  1.25  p.  m.  small  torn  cumulus  clouds  passed 
rapidly  southwest  to  northeast  in  advance  of  the  cloud  of  dust.  This 
was  followed  by  a  stratus  layer  which  by  1.30  p.  m.  covered  the  entire 
sky.  At  1.45  p.  m.  the  stratus  clouds,  moving  southwest  to  northeast, 
began  to  break  away.  Between  the  open  spaces  strato-cunuilus  clouds 
were  visible  above  moving  from  west  to  east.     (See  Fig.  154.) 

29 


440 


THE    CLIMATE    OF   BALTIMORE 


The  first  peals  of  thunder  were  heard  at  1.23  p.  m.,  coming  from  the 
southwest.  The  electrical  display  was  brilliant  during  the  height  of  the 
storm.  The  last  thunder  was  heard  at  2.25  p.  m.  Light  rain  began  at 
1.27  p.  m.,  changing  to  a  heavy  shower  at  1.29  p.  m. ;  the  rain  moved  in 
dense  sheets  from  southwest  to  northeast.  In  the  meantime  the  wind 
was  increasing  in  velocity;  at  1  p.  m.  it  registered  17  miles  per  hour, 


Fig.  153.— The  Thunderstorm  of  July  20,  1902. 


remaining  at  this  velocity  until  1.20  p.  m.  In  the  next  five  minutes  the 
velocity  suddenly  increased  to  46  miles,  and  at  1.31  p.  m.  it  blew  at  the 
excessive  rate  of  75  miles  per  hour  from  the  west.  Coincident  with  the 
sudden  increase  in  the  wind  velocity  the  pressure  rose  nearly  a  tenth  of 
an  inch  in  the  course  of  a  few  minutes,  while  the  temperature  fell  as 
rapidly  from  94°  to  69°  and  the  rain  fell  in  torrents  for  a  few  minutes. 
The  wind  rapidly  fell  and  by  2  p.  m.  had  regained  its  early  morning 
velocity  of  5  to  6  miles  per  hour.     The  pressure  regained  in  half  an 


MARYLAND   WEATHER   SERVICE 


441 


hour  the  height  at  which  it  stood  before  the  sudden  rise,  and  then 
continued  to  fall  slowly  until  about  8  p.  m.  The  temperature  rose 
rapidly  after  the  sudden  fall,  recording  84°  at  3.30  p.  m.,  maintaining 


MDT. 


6    A. 


NOON 


6    P. 


MDT. 


Fig.  154.— The  Thunderstorm  of  July  20,  1902. 

this  reading  approximately  until  G  p.  m.,  when  it  fell  rapidly  to  76° 
shortly  after  8  p.  m. 

An  hour  or  so  before  the  sudden  fluctuations  in  wind  velocity,  tem- 
perature, and  pressure  recorded  above,  the  wind  veered  from  southwest  to 


443 


THE    CLIMATE    OE   BALTIMORE 


MARYLAND   WEATHER    SERVICE  443 

west.  During  the  afternoon  and  evening  the  direction  alternated  fre- 
quently between  the  west  and  southwest. 

The  storm  was  of  short  duration;  the  interval  between  the  first  and 
last  thunder  heard  being  about  an  hour.  Its  greatest  intensity  was 
reached  within  20  minutes  after  the  first  peal  of  thunder  was  heard. 
The  rainfall  began  at  1.27  p.  m.  and  ended  at  1.55  p.  m.,  with  a  total 
precipitation  of  a  little  over  half  an  inch. 

During  the  morning,  cirrus,  cirro-stratus,  and  alto-cumulus  clouds 
were  observed  passing  across  the  sky  from  west  to  east  with  considerable 
rapidity.  A  cloud  of  dust  preceded  the  thunderstorm,  carrying  leaves, 
paper  and  other  light  objects  high  up  into  the  air.  Just  preceding  and 
during  the  storm  the  humidity  was  fully  40  per  cent  higher  than  at 
the  8  a.  m.  observation,  ^o  portion  of  the  city  was  free  from  damage 
caused  by  the  storm,  although  north  Baltimore  seemed  to  have  sufi'ered 
less  severely  than  other  sections. 

The  storm  described  above  caused  considerable  damage  in  all  parts  of 
Maryland,  though  most  of  the  loss  of  life  and  property  occurred  in  the 
vicinity  of  Baltimore.  A  special  effort  was  made  at  the  time  to  trace  the 
path  of  the  storm  across  the  state.  Co-operative  observers  in  Maryland, 
Virginia,  West  Virginia,  and  Delaware  were  requested  to  report  accurately 
ihe  time  of  day  when  the  first  thunder  was  heard  in  their  respective 
localities.  Eeplies  were  received  from  about  150  observers,  making  it 
possible  by  charting  the  recorded  times  upon  a  map  and  joining,  by  a 
line,  localities  over  which  the  storm  passed  at  about  the  same  hour,  to 
follow  the  storm  from  West  Virginia  eastward  to  the  Atlantic  coast. 

The  accompanying  chart  shows  the  hourly  rate  at  which  the  storm 
travelled.  In  Central  West  Virginia  the  storm  began  at  10  a.  m.  From 
West  Virginia  it  passed  into  Maryland  by  way  of  Washington  county 
between  noon  and  1  p.  m.  It  then  advanced  with  an  irregular  wave 
front  eastward  to  the  Chesapeake  Bay  by  2  p.  m.     (See  Fig.  155.) 

The  storm  front  then  moved  in  a  southeast  direction,  passing  beyond 
the  limits  of  Maryland  in  Worcester  County  between  6  p.  m.  and  7 
p.  m.  The  total  distance  traversed  by  the  storm  from  10  a.  m.  to  6 
p.  m.  was  about  200  miles,  or  at  the  rate  of  25  miles  per  hour.     The 


444 


THE    CLIMATE    OF   BALTIMORE 


irregular  form  of  the  storm  front  and  its  var3dng  rate  of  progress  across 
the  state  are  clearh^  shown  in  the  chart  (Fig.  153). 

THE  THUNDERSTORM  OF  JULY   3,   1902. 

At  8  p.  m.  of  July  3,  1902,  a  moderate  barometric  depression  was 
centered  over  Lake  Ontario,  while  the  western  edge  of  an  extensive  area 
of  high  pressure  covered  the  South  Atlantic  states  and  extended  far  out 


Fig.  156.— The  Thunderstorm  of  July  3,  1902. 

over  the  Atlantic  Ocean.  The  depression  was  attended  by  moderate 
rains  in  New  England,  the  Lake  region,  and  the  Middle  Atlantic  states. 
In  the  vicinity  of  Baltimore,  as  shown  by  official  records,  the  day  was 
partly  cloudy  and  oppressive;  a  film  of  cirro-stratus  covered  the  sky  from 
early  morning,  through  which  the  sun  shone  with  great  intensity.  At 
4  p.  m.  stratus  clouds  were  observed  moving  rapidly  from  the  north  and 
northeast.     During  the  morning  and  early  afternoon  a  light  to  fresh 


MARYLAND   WEATHER   SERVICE 


445 


breeze  blew  from  the  southwest;  at  3.15  p.  m.  the  direction  of  the  wind 
changed  to  west,  and  its  force  increased  to  brisk  for  a  brief  time.  B}- 
4  p.  m.  the  velocity  of  the  wind  began  to  increase  rapidW,  the  clouds 


MDT, 


6    A. 


NOON 


6    P. 


MDT 


Fig.  1.37.— The  Thunderstorm  of  July  3,  1902. 


maintaining  their  original  direction.  At  4.25  p.  m.  the  wind  shifted 
to  the  northwest,  blowing  with  increasing  velocity,  and  attaining  a  maxi- 
mum rate  of  34  miles  per  hour  at  4.29  p.  m.  At  the  same  time  great 
numbers  of  cumulus  clouds  were  rapidly  carried  across  the  sky  from  the 


446  THE    CLIMATE    OF   BALTIMORE 

north-northwest.  The  storm  front  moved  with  great  rapidity  to  the 
southeast,  the  usual  dust  cloud  marking  the  advance.  The  squall-wind  car- 
ried light  objects  high  into  the  air.  A  number  of  lives  were  lost  during 
the  squall,  while  considerable  property  was  damaged,  and  many  trees 
were  uprooted. 

Eain  began  at  4.35  p.  m.  and  continued  until  4.50  p.  m.,  the  amount 
being  0.04  inch.  On  the  arrival  of  the  stormfront,  marked  changes  in 
the  barometer  and  thermometer  were  noted.    (See  accompanying  diagram.) 

The  barometer  fell  rapidly  throughout  the  day  until  shortly  after  4 
p.  m.,  while  the  thermometer  rose  from  66°  at  5  a.  m.  to  96°  at  4  p.  m. 
With  the  sudden  change  of  wind  at  4.30  p.  m.  from  southwest  to  west  and 
northwest,  there  was  an  abrupt  rise  of  nearly  a  tenth  of  an  inch  in  the 
barometer.  The  temperature  fell  as  abruptly  from  96°  to  74°,  while 
the  wind  rose  from  12  miles  to  32  miles  per  hour.  The  change  was 
accompanied  by  a  sharp  shower  of  rain  of  a  few  minutes'  duration. 

The  barometer  lost  a  part  of  the  sudden  rise  during  the  following 
hour  and  then  continued  to  rise  slowly  during  the  balance  of  the  day. 
The  temperature  remained  low  after  the  storm.  No  thunder  and  light- 
ning were  noted  in  connection  with  this  storm  while  it  passed  over  Balti- 
more. Electrical  displays  were,  however,  reported  from  many  parts  of 
the  state  on  this  da3^  The  progressive  changes  were  all  characteristic 
of  a  well  defined  thunderstorm. 

THE  THUNDERSTORM  OF  JULY  12,  1904. 

There  is  a  type  of  pressure  distribution  which  invariably  gives  rise  to 
numerous  and  severe  thunderstorms  and  squalls.  It  is  represented  in  the 
accompanying  chart  showing  the  general  weather  conditions  at  8  p.  m. 
of  July  12,  1904.  It  is  the  V-shaped  depression  referred  to  in  a  preced- 
ing paragraph  in  connection  with  a  discussion  of  storm  types.  In  this 
instance  the  "  squall  line  "  is  particularly  well  defined  by  a  long  narrow 
band  of  thunderstorms  and  rain  at  or  near  8  p.  m.,  extending  from  the 
St.  Lawrence  Valley  southward  through  New  York,  New  Jersey,  Eastern 
Pennsylvania,  Eastern  Maryland,  Delaware,  and  along  the  coast  south- 
ward to  Florida.     (See  Fig.  158.) 


MARYLAND    WEATHER    SERVICE 


447 


Fig.  158.— The  Thunderstorm  of  July  12,  1904. 


THE   TORNADO   OF   JULY    12,   1903. 

The  Middle  Atlantic  states  are  rarely  visited  by  tornadoes.  There  are 
descriptions  of  such  storms  on  record  in  the  annals  of  Baltimore  weather, 
but  the  storms  were  of  a  mild  type  of  tornado  so  far  as  can  be  judged 
by  local  descriptions.  Tornadoes  occur  under  general  conditions  similar 
to  those  which  give  rise  to  thunderstorms  and  squalls.  They  differ, 
however,  from  the  latter  in  the  character  of  the  atmospheric  circulation 
within  the  storm,  in  their  greater  destructiveness,  and  in  the  fact  that  tliey 
are  more  restricted  in  the  area  of  their  activity.  The  air  within  a 
thunderstorm  moves  about  a  horizontal  axis,  while  within  a  tornado  the 
circulation  is  about  a  vertical  axis.  The  thunderstorm  moves  eastward 
with  the  general  cyclone  with  a  long  wave  front  many  miles  in  length, 
the  tornado  moves  along  with  the  general  storm  in  the  form  of  a  vertical 
column  of  limited  extent,  generally  less  than  half  a  mile  in  diameter. 


448 


THE    CLIMATE    OF   BALTIMORE 


HIGH 


Fig.  159.— The  Tornado  of  July  12,  1903  (8  a.  m.). 


Pig.  160.— The  Tornado  of  July  12.  1903   (8  p.  m.). 


MARYLAND   WEATHER   SERVICE  449 

usually  recognizable  as  a  downward  extension  of  the  cloud  mass  which 
generally  reaches  to  the  ground,  but  sometimes  dangles  in  mid-air  like 
the  loose  end  of  a  suspended  rope. 

The  storm  of  July  13,  1903,  as  it  passed  over  Baltimore,  had,  from  the 
best  information  obtainable  from  eye  witnesses,  many  of  the  traits  of 
the  real  tornado,  although  it  is  frequently  difficult  to  distinguish  between 
a  mild  type  of  tornado  and  an  intensely  developed  thunderstorm. 

The  general  weather  conditions  were  favorable  for  the  production  of 
local  storms  over  a  large  portion  of  the  Atlantic  and  Gulf  Coast  states 
and  the  Ohio  Valley.  Cloudy  and  unsettled  weather  prevailed  in  the 
sections  named  at  8  a.  m.  of  the  12th.  Thunderstorms  were  reported 
from  many  stations  for  the  preceding  twelve  hours.  There  was  an  area 
of  high  pressure  in  the  northwest,  and  a  barometric  depression  over  the 
Gulf  of  St.  Lawrence,  with  a  secondary  depression  forming  over  the 
Lower  Mississippi  Valley.  The  temperature  conditions  were  nearly  nor- 
mal, but  the  humidity  was  high.  The  prevailing  wind  direction  at 
stations  in  the  Atlantic  Coast  states  was  from  the  southwest  and  light 
in  force,  excepting  in  the  South  Atlantic  states,  where  they  were  fresh 
to  brisk.  During  the  succeeding  24  hours  the  secondary  depression  had 
developed  and  moved  rapidly  northeastward  over  Maryland,  the  center 
being  over  Massachusetts  at  8  a.  m.  of  the  13th,  accompanied  by  heavy 
rains  and  severe  local  storms  in  the  South  Atlantic  states,  and  near  the 
coast  in  the  Middle  Atlantic  and  New  England  states.  The  following 
heavy  rainfalls  were  reported  for  the  preceding  24  hours  at  8  a.  m.  of 
the  13th:  Baltimore,  Md.,  3.98  inches;  Washington,  D.  C,  3.02  inches; 
Atlantic  City.  N".  J.,  1.74  inch.     (See  Figs.  159  and  160.) 

The  records  of  the  local  office  of  the  United  States  Weather  Bureau 
contain  the  following  account  of  the  storm  as  recorded  by  the  official 
and  other  observers: 

On  July  12  thunderstorms  and  heavy  rainfall  were  general  throughout  the 
section.  At  Baltimore  the  storm  at  its  height  developed  destructive  features 
over  a  limited  area.  A  funnel-shaped  cloud,  peculiar  to  the  tornado  was 
clearly  in  evidence  a  few  minutes  after  noon,  and  the  narrow  path  pursued 
by  this  cloud  was  also  the  path  of  devastation.  The  cloud  moved  from  west 
to  east,  descending  to  the  house-tops  at  two  points  within  the  city,  leaving  a 


450  THE    CLIMATE    OF    BALTIMORE 

wide  gap  of  comparatively  slight  loss  between;  it  evidently  struck  the  ground 
again  a  short  distance  beyond  the  city  proper,  judging  from  the  local  damage 
there,  and  then  disappeared,  as  far  as  surface  traces  were  concerned.  Re- 
ports from  parts  of  Kent  County,  however,  would  indicate  that  the  storm 
crossed  the  Bay  and  moved  over  the  Eastern  Shore,  for  in  that  county  a  nar- 
row area  was  visited  by  destructive  winds. 

The  following  is  the  special  report  of  Mr.  James  S.  Harris,  the  co-operative 
observer  at  Coleman,  regarding  this  visitation:  "About  1  p.  m.  an  angry 
black  cloud  came  suddenly  over,  causing  a  darkness  as  of  twilight,  accom- 
panied by  a  cyclone  and  hail.  Wheat  in  shock  and  stack  was  blown  about, 
trees  blown  down,  and  houses  wrecked."  In  his  use  of  the  word  "  cyclone  " 
the  writer  doubtless  intended  to  describe  a  tornado,  a  confusion  of  terms  so 
frequently  met  with  in  popular  accounts  of  storms  of  this  class.  Baltimore 
and  Coleman  seem  to  have  marked  the  extreme  limits  of  tornado  winds, 
although  the  thunderstorms  were  more  or  less  severe  at  many  other  points 
on  the  same  day. 

In  Baltimore  the  first  area  visited  embraced  much  of  the  1700  blocks  of 
Fulton  Avenue,  Mount  Street,  and  Calhoun  Street;  here  a  funnel-shaped 
cloud  was  distinctly  observed  by  a  number  of  the  residents,  but  no  definite 
account  of  its  manner  of  formation  was  obtained  beyond  this.  In  the  second 
district,  which  extended  from  Eager  Street  and  Broadway  eastward  for  six 
blocks,  with  a  width  varying  from  two  blocks  to  less  than  a  block,  the  damage 
was  greater  and  the  information  obtained  was  more  explicit.  A  heavy  storm 
cloud  approached  from  the  northwest  and  another  from  the  southwest;  they 
apparently  merged  at  Eager  Street  and  Broadway,  where  the  destruction 
abruptly  began.  The  funnel-shaped  cloud  was  seen  by  many,  and  a  loud 
roaring  sound  was  followed  by  almost  complete  darkness  as  the  storm  burst. 
The  upper  cloud  mass  was  distinguishable,  however,  with  its  narrowing 
extension  downwards,  the  latter  appearing  to  lag  slightly  behind  the  mass 
above  in  its  movement  eastward.  The  whole  travelled  with  almost  incredible 
velocity,  only  a  few  seconds  elapsing  between  the  time  the  cloud  descended 
to  the  house-tops  at  Eager  Street  and  Broadway  and  the  time  when  it  rose 
into  the  air  again  six  blocks  to  the  eastward. 

In  both  districts  the  nature  of  the  destruction  pointed  clearly  to  the  fact 
that  the  city  had  been  visited  by  a  tornado.  In  some  of  the  wrecked  houses 
the  walls  were  blown  outward,  as  though  by  sudden  expansion  of  confined 
air  within,  although  fully  as  many  fell  inward.  In  one  case  the  four  walls 
had  bulged  outward,  and  the  roof  lay  within,  about  half-way  down  to  the 
fioor  of  the  second  story,  while  not  far  off  roofs  had  been  lifted  high  into  the 
air  and  carried  a  block  and  a  half  away  before  being  deposited  in  an  alley 
to  the  rear.  In  all  several  hundred  houses  were  unroofed  or  otherwise  badly 
wrecked.  The  money  loss  was  estimated  at  $200,000;  happily  there  was  no 
loss  of  life,  although  one  man  was  seriously  hurt  by  falling  walls,  and  nu- 
merous narrow  escapes  from  injury  were  reported. 

At  the  Weather  Bureau  Office,  about  a  mile  and  a  half  away,  no  damage 
occurred.  The  self-registering  instruments,  while  presenting  interesting 
records,   do   not   adequately   portray   the   conditions   as   they   existed   at   the 


MARYLAND    WEATHER    SERVICE 


451 


centers  of  severe  damage.  The  rainfall  at  the  station  was  unusually  heavy; 
2.87  inches  fell  in  33  minutes,  from  12.04  p.  m.  to  12.37  p.  m.  The  following 
maximum  falls  were  tabulated: 

Greatest  amount 


ount  in     5 

minutes, 

O.SO   inc 

"    10 

1.35       ' 

"    15 

1.92       ' 

"    20 

2.22       ' 

"     25 

2.46       ' 

"     30 

2.75       ' 

"     35 

2.87       ' 

Further  details  of  rainfall  in  connection  with  this  storm  are  given  on 
pages  212  and  213.  The  rate  of  rainfall  in  the  districts  of  greatest  storm  loss 
must  have  been  much  heavier.  The  streets  were  running  streams  of  water, 
and  cellars  were  entirely  filled  within  a  few  minutes. 

At  the  Baltimore  station  the  wind  was  comparatively  high  from  12.04 
p.  m.  to  12.15  p.  m.,  and  brisk  to  light  thereafter.  The  maximum  velocity 
was  46  miles  per  hour  at  about  12.05  p.  m.  The  wind  direction  veered 
through  nearly  all  of  the  points  of  the  compass  during  the  storm,  as  shown 
by  the  following  record: 

Noon  to  12.05  p.  m Southwest. 


12.05    • 

'    12.15 

12.15    ' 

'    12.20 

12.20    ' 

'    12.25 

12.25    ' 

'    12.45 

12.45    ' 

'      1.00 

.West  (mostly). 
.Northwest. 
.North. 
.Northeast. 
.East   (mostly). 


There  was  a  sharp  fall  of  about  15°  in  temperature  at  the  beginning  of 
the  storm,  but  at  the  office  of  the  Weather  Bureau  the  variation  in  atmos- 
pheric pressure  was  very  slight.  The  only  noteworthy  feature  of  the  pressure 
curve  was  a  small  but  sudden  rise  of  about  0.05  inch,  characteristic  of  severe 
thunderstorms  accompanied  by  hail. 

The  general  weather  conditions  on  the  day  of  the  storm  are  recorded 
as  follows  in  the  local  office  of  the  United  States  Weather  Bureau : 

Cloudy  day.  Not  so  warm.  Atmosphere  very  oppressive  in  forenoon; 
pleasant  afterwards.  Maximum,  85°  at  11  a.  m.;  temperature  then  fell  to 
70°  at  noon,  rose  to  74°  at  4  p.  m.,  fell  to  69°  at  8  p.  m.,  and  remained 
stationary  until  midnight.  Sky  partly  covered  at  dawn,  became  overcast  by 
9  a.  m.  with  alto-stratus  clouds.  At  11  a.  m.  a  dark  low-lying  cloud  mass 
appeared  on  the  northern  and  western  horizons,  moving  slowly.  Shortly 
before  noon,  the  movement  of  the  cloud  mass  increased  very  rapidly,  and  the 
sky  became  covered  in  a  few  minutes,  continuing  so  the  rest  of  the  day. 
This  movement  of  the  clouds  was  followed  by  a  terrific  thunderstorm,  thunder 
being  first  heard  at  11.48  a.  m.,  continuing  all  the  afternoon  at  intervals, 
being  last  heard  at  6.45  p.  m.,  becoming  recognizable  as  a  second  storm  at 


452  THE    CLIMATE    OF   BALTIMORE 

about  6  p.  m.  The  first  storm  moved  from  west  to  east,  the  second  passed 
from  south  to  north.  A  trace  of  rain  fell  in  the  early  morning.  The  periods 
of  rainfall  during  the  day  were  as  follows: 

12  noon  to  1.10  p.  m. 

1.45  p.  m.  to  2.10  p.  m. 

2.50  p.  m.  to  3.20  p.  m. 

4.45  p.  m.  to  5.05  p.  m. 

5.40  p.  m.  to  7.50  p.  m. 

9.25  p.m.  to  9.40  p.m. 

The  rainfall  was  excessive  from  12.07  p.  m.  to  12.42  p.  m.  (2.87  inches), 
and  heavy  from  6.20  p.  m.  to  6.40  p.  m.  (0.72  inch) ;  the  total  amount  for  the 
day  was  3.90  inches.  A  light  southeast  wind  before  noon  shifted  suddenly  to 
west  at  noon  with  increased  force,  being  brisk  to  high  from  12.02  p.  m.  until 
12.27  p.  m.,  with  a  maximum  velocity,  at  the  station,  of  46  miles  from  the 
west  at  12.07  p.  m.  The  winds  were  light  and  variable  the  rest  of  the  day, 
mostly  from  the  north. 

WATERSPOUTS. 

Waterspouts  are  in  their  mode  of  formation  and  in  their  external 
appearance  similar  to  tornadoes.  In  extent,  however,  they  are  much  more 
restricted,  while  they  do  not  compare  with  the  tornado  in  destructive 
power.  They  are  of  comparatively  infrequent  occurrence  and  it  is  not 
often  that  they  are  observed  at  close  range  by  an  intelligent  observer, 
hence  the  following  description  is  of  special  interest: 

Early  in  April,  1902,  Captain  Fergus  Ferguson  of  the  British  S.  S.  Hestia 
left  Baltimore  for  one  of  the  Cuban  ports.  On  April  4,  towards  sunset,  while 
off  Hatteras,  the  Captain  observed  several  waterspouts  in  process  of  forma- 
tion at  a  distance  of  300  to  400  yards  to  windward.  The  largest  of  these, 
and  the  only  one  completely  formed,  seemed  to  be  headed  directly  for  the 
Hestia.  The  Captain  at  first  attempted  to  change  his  course  sufficient  to 
avoid  running  into  it,  but  soon  discovered  that  this  could  not  be  done. 
Giving  orders  for  all  on  deck  to  go  below,  he  remained  until  the  spout  was 
close  upon  his  ship,  and  then  hastily  sought  a  place  of  safety.  In  a  moment 
he  heard  a  deafening  roar  which  was  quickly  followed  by  strong  gusts  of 
wind  and  a  sudden  shock  as  the  spout  struck  amidships  and  passed  over  the 
deck  towards  the  stern.  The  Captain  reappeared  upon  deck  in  time  to  see 
two  tarpaulins,  which  had  covered  the  hatches,  and  a  plank  8  feet  long  by 
10  inches  wide,  high  in  the  air,  while  his  log  line  with  log  attached  extended 
straight  up  into  the  air  to  a  distance  of  about  40  feet.  Beyond  the  loss  of 
the  lighter  movable  objects  on  deck  and  a  temporary  feeling  of  apprehension 
no  harm  was  done. 


MARYLAND   WEATHER   SERVICE  453 

When  first  seen,  the  waterspout  was  incomplete.  A  portion  of  the  cloud 
dipped  down  from  the  general  cloud  level  of  about  2000  feet,  while  at  the 
same  time  a  column  of  water  was  apparently  rising  from  the  surface  of  the 
ocean  just  below.  At  an  elevation  of  between  200  and  300  feet  the  ascending 
water  column  and  the  descending  cloud  column  met.  The  diameter  of  the 
spout  was  approximately  the  width  of  the  Hestia,  or  between  40  and  50  feet. 
Within  the  column  there  was  a  dark  core,  almost  black,  with  a  diameter  of 
about  2  feet.  Captain  Ferguson  did  not  clearly  recall  evidences  of  a 
whirling  motion,  but  a  strong  upward  movement  is  clearly  indicated  by  the 
facts  noted  above.  No  reference  was  made  to  any  considerable  quantity  of 
water  being  shipped  as  the  waterspout  passed  over  the  vessel,  a  fact  which 
would  indicate  that  the  lower  portion  of  the  column  was  composed  mostly  of 
spray  carried  up  by  the  strong  wind  from  the  surface  of  the  ocean. 

At  the  time  of  occtirrence  of  the  waterspout  the  Hestia  was  near  the 
center  of  a  shallow  but  well  defined  barometric  depression  just  off  the 
coast  of  North  Carolina.  The  general  storm  was  moving  slowly  up  the 
coast.  A  ridge  of  high  pressure  extended  from  the  St.  Lawrence  Valley 
southwestward  to  the  West  Gulf  states.  The  winds  along  the  coast  from 
New  England  to  North  Carolina  were  northerly. 

Summer  Hot  Spells. 

One  of  the  most  characteristic  features  of  our  summer  season  is  the 
frequent  recurrence  of  a  longer  or  shorter  series  of  excessively  warm 
days.  No  summer  season  is  entirely  free  from  them,  although  at  times 
they  are  not  frequent  enough  or  intense  enough  to  cause  comment.  These 
periods  vary  greatly  in  length  and  in  the  frequency  of  their  occurrence. 
When  the  atmosphere  is  comparatively  dry,  high  temperatures  may  be 
endured  without  great  personal  discomfort.  A  high  humidity  combined 
with  even  moderately  high  temperatures  is  the  cause  of  most  of  the 
unfavorable  comment  upon  the  summer  weather  of  the  Middle  Atlantic 
coast  states. 

In  the  usual  course  of  summer  events  cyclones  and  anti-cyclones, 
though  not  as  frequent  as  in  winter  or  spring,  and  not  so  intense,  are 
3'et  sufficiently  frequent  in  their  passage  across  the  northern  portion  of 
the  country  to  maintain  a  fairly  well  mixed  atmosphere  and  thus  prevent 
the  accumulation  of  excessively  heated  air  near  the  surface  of  the  earth. 
At  times,  however,  we  have  a  comparatively  stationary  system  of  cyclones 


454  THE    CLIMATE    OF   BALTIMORE 

and  anti-cyclones  with  a  small  gradient,  which  may  persist  with  very 
little  change  in  position  for  many  days.  Such  a  system  is  of  frequent 
occurrence  in  the  summer  season  in  the  United  States.  An  area  of  high 
pressure  settles  over  the  South  Atlantic  states,  or  over  the  Atlantic  Ocean 
with  an  extension  covering  the  South  Atlantic  states,  while  a  barometric 
depression  rests  over  the  Missouri  Valley  or  the  eastern  slope  of  the 
Rocky  Mountains.  While  this  distribution  of  pressure  continues  there  is 
a  steady  flow  of  warm  dry  southeast  to  southwest  winds  over  the  Middle 
Atlantic  states.  If  in  addition  the  gradient  is  small,  or  the  South 
Atlantic  high  area  moves  over  the  Middle  Atlantic  states,  the  winds 
become  very  light  while  the  clear  skies  permit  uninterrupted  sunshine. 
The  sluggish  movement  of  the  atmosphere  together  with  the  unobstructed 
insolation  permits  the  accumulation  of  excessively  heated  layers  of  atmos- 
phere at  the  surface  of  the  earth.  Sometimes  these  conditions  will  per- 
sist for  two  or  three  weeks  before  the  cyclonic  and  anti-cyclonic  systems 
begin  to  move  eastward  in  their  accumstomed  paths  and  bring  about  a 
change. 

THE    SUMMER    OF    1900. 

The  summer  of  1900  was  probably  the  warmest  in  the  annals  of 
Middle  Atlantic  states  weather.  The  temperatures  at  the  beginning  of 
the  season  were  about  normal,  June  averaging  but  0.2°  per  day  below  the 
average  of  30  years.  Beginning  with  July  the  average  monthly  tepera- 
tures  remained  far  above  their  normal  seasonal  values  until  the  close 
of  November,  the  departures  from  the  normal  increasing  steadily  from 
July  to  September  and  then  decreasing  slowly  to  November. 

July    -f2.0° 

August     +4.7° 

September    -}-5.1° 

October    +4.5° 

November     +3.5° 

July,  1900.  During  the  first  two  days  in  July  northerly  winds  pre- 
vailed in  Maryland,  accompanied  by  a  cool  morning  temperature  of  about 
60°  in  the  central  and  eastern  portions  of  the  state.  On  the  Allegany 
plateau  the  night  temperatures  were  as  low  as  40°.    The  maximum'  after- 


MARYLAND   WEATHER   SERVICE  455 

noon  temperatures  were  about  85°.  On  the  whole,  these  days  were  several 
degrees  cooler  than  the  normal  for  the  season.  On  the  3d  the  temperature 
began  to  rise  rapidly.  At  Baltimore  the  maximum  was  92°,  and,  with 
the  exception  of  four  or  five  days  during  which  the  maximum  registered 
in  the  eighties,  the  afternoon  temperatures  remained  well  above  90°  until 
the  21st  of  the  month.  From  the  22d  to  the  31st  the  maximum  readings 
ranged  between  80°  and  91°.  The  hot  spell  culminated  in  temperatures 
of  100°  on  the  16th  and  17th.  These  temperatures,  occurring  at  Balti- 
more, fairly  represent  the  conditions  that  prevailed  in  the  central,  eastern, 
and  southern  portions  of  the  state.  In  the  valleys  of  Washington  and 
Allegany  counties  the  figures  are  somewhat  higher.  Thus,  at  Hagers- 
town,  a  reading  of  105°  was  recorded  on  the  16th;  at  Hancock,  105°  on 
the  15th,  16th,  and  17th;  at  Green  Spring  Furnace,  106°  on  the  17th,  the 
highest  in  the  state.  Within  a  very  restricted  area  Mar3dand  offers  a 
great  variety  of  climatic  conditions.  On  the  Allegany  plateau,  in  Garrett 
Count}^  the  thermometer  did  not  register  above  92°  during  the  entire 
month,  and  then  only  on  one  or  two  days. 

The  temperatures  here  indicated  are  all  shade  temperatures,  that  is, 
they  were  registered  by  thermometers  placed  in  standard  shelters  which 
protect  the  instruments  from  the  direct  rays  of  the  sun,  or  reflected  rays 
from  neighboring  objects,  but  are  so  constructed  as  to  permit  of  free  circu- 
lation of  the  air.  Thermometers  exposed  to  the  direct  rays  of  the  sun  at 
Chase,  in  Baltimore  County,  and  at  Chewsville,  in  Washington  County, 
gave  an  average  maximum  of  104°  on  13  days,  with  an  absolute  maximum 
of  110°.  Such  temperatures,  are,  however,  not  unusual  with  thermome- 
ters so  exposed.  The  average  number  of  days  with  a  maximum  tempera- 
ture of  90°  or  above  in  July  at  Baltimore,  based  on  the  30  years  of  care- 
fully kept  records  of  the  U.  S.  Weather  Bureau,  is  9  days.  Their  fre- 
quency has  varied  from  a  total  absence  in  1891,  to  18  in  1876.  During 
July,  1900,  there  were  15  such  days  at  Baltimore,  17  at  Washington,  18 
at  Hagerstown,  19  at  Laurel,  21  at  Taneytown,  and  27  at  Hancock. 
Frostburg  had  but  5,  Grantsville  and  Deer  Park  2  each,  while  at  Sunn)^- 
side,  Garrett  County,  there  was  but  one  day.  The  average  daily  maximum 
temperature  at  Baltimore  during  these  15  days  was  95°;  the  normal 
30 


456  THE    CLIMATE    OF    BALTniORE 

average  for  the  same  period  is  86°,  showing  a  daily  excess  of  9°.  These 
excessive  temperatures  caused  the  average  daily  temperatures  for  the 
entire  month  in  Maryland  and  Delaware  to  be  2.5°  to  8°  above  the 
normal  value  for  the  season. 

The  weather  conditions  whicli  usually  accompany  hot  spells  were  pres- 
ent in  a  marked  degree  during  July,  1900.  The  skies  were  remarkably 
clear ;  the  winds  were  prevailingly  southwest,  and  generally  light  in  force ; 
the  rainfall  was  deficient  in  quantity  and  frequency.  The  records  from 
over  50  stations  in  Maryland,  Delaware,  and  the  District  of  Columbia 
show  an  average  of  17  clear,  11  partly  cloudy,  and  3  cloudy  days.  The 
average  conditions  at  Baltimore,  derived  from  30  years  of  observations, 
are  10  clear,  13  partly  cloudy,  and  8  cloudy  days.  The  winds  were  almost 
constantly  from  the  south  or  southwest  while  the  high  temperatures  pre- 
vailed. At  Baltimore  they  were  from  the  southeast,  south,  or  southwest 
during  20  days  out  of  the  31.  The  average  hourly  velocity  was  but  4.6 
miles,  approximately  the  lowest  in  25  years,  during  July,  while  the  high- 
est velocity  for  the  month  was  only  18  miles,  the  smallest  maximum 
recorded  at  Baltimore.  Scattered  showers  fell  from  the  3d  to  the  9th; 
on  the  12th  and  30th  rainfall  was  general  throughout  the  states  of  Mary- 
land and  Delaware;  during  the  period  from  the  17th  to  26th  local 
showers  were  frequent.  With  but  few  exceptions  the  total  rainfall  for 
the  month  Avas  decidedly  below  the  average.  Baltimore  had  but  1.31  inch 
and  Washington,  D.  C,  but  1.25  inch,  whereas  the  average  rainfall  for 
July  in  this  vicinity  is  about  4.50  inches.  The  relative  humidity  during 
the  period  of  intense  heat  was  somewhat  below  the  average  for  the  month, 
a  circumstance  affording  some  cause  for  thankfulness. 

While  suffering  the  discomforts  of  an  intense  spell  of  warm  weather, 
we  are  apt  to  overestimate  its  severity  as  compared  with  those  experienced 
in  the  past.  Statistics,  however,  support  the  assertion  that  this  July  hot 
spell  was  one  of  the  most  trying  on  record  in  our  vicinity.  It  is  always 
difficult  to  make  just  comparisons  in  dealing  with  weather  conditions. 
We  feel  hot  and  uncomfortable  and  look  for  the  cause  in  high  tempera- 
tures alone,  but  do  not  always  find  them  as  high  as  expected.  The  ele- 
ment of  personal  discomfort  is  due  to  certain  combinations  of  tempera- 


MARYLAND    WEATHER    SERVICE  457 

ture,  humidity,  and  air  movement,  and  we  have  no  single  set  of  values 
to  express  this  element,  ^^'e  can  and  do  measure  accurately  the  tempera- 
ture, the  humidity,  and  the  wind  direction  and  velocity,  each  separately. 
Upon  these  figures  we  must  base  our  judgment  of  the  severity  of  any  disa- 
greeable period  of  weather.  Since  1871,  the  date  of  the  establishment  of 
the  Weather  station  at  Baltimore,  the  number  of  days  in  July  with  a  max- 
imum temperature  of  90°  or  above  has  exceeded  15  but  twice.  In  1878 
there  were  16  such  days  with  an  absolute  maximum  of  98° ;  the  average 
of  the  maximum  temperatures  was  92.5°  as  compared  with  95°  in  1900. 
The  average  relative  humidity  was  the  same  in  both  instances,  namely  63 
per  cent.  The  average  daily  wind  movement  was  greater  in  1878  than  in 
1900,  128  miles  in  the  former  and  117  miles  in  the  latter  period.  In 
1876  there  were  18  consecutive  days  with  an  average  maximum  of  93°, 
and  an  absolute  maximum  of  99°;  the  average  relative  humidity  during 
this  period  was  63  per  cent;  while  the  average  wind  movement  was  125 
miles  per  day.  As  a  result  of  this  comparison  with  the  two  most  con- 
spicuous rivals  for  notoriety,  we  find  that  the  hot  spell  of  July,  1900, 
was  but  little  shorter  in  duration ;  that  the  humidity  was  as  high ;  that 
the  average  temperature  was  fully  2°  higher;  and  that  the  wind  velocity, 
a  powerful  element  of  relief  on  a  muggy  da}^,  was  less. 

August,  1900.  According  to  statistics  of  the  Baltimore  Health  Officer 
there  were  30  deaths  during  August  due  directly  to  sunstroke,  and  32  in 
addition  due  to  excessive  heat  as  a  secondary  cause.  When  we  come  to 
examine  the  record  of  weather  conditions  during  this  period,  and  compare 
it  with  the  hot  spells  of  the  past,  we  find  nothing  to  equal  it  in  intensity 
since  the  establishment  of  the  Weather  Bureau  Station  in  Baltimore  in 
1871. 

Baltimore  has  on  an  average  five  days  in  August  with  a  temperature  of 
90°  or  above,  with  a  maximum  in  the  past  of  98°.  In  August,  1900, 
there  were  17  such  days,  with  a  maximum  of  100°,  while  this  maximum 
was  practically  maintained  for  six  consecutive  days.  Temperatures  were 
even  higher,  and  hot  days  more  frequent  at  other  points  in  Maryland  and 
Delaware.  Thus,  in  Washington  County  there  were  20  days  with  a 
maximum  temperature  of  90°  or  above,  with  an  absolute  maximum  of 


458  THE    CLIMATE    OF   BALTIMORE 

103°  at  Hancock.  The  highest  temperature  recorded  within  the  two 
states  was  10-i°  at  Millsboro,  Delaware,  on  the  14th. 

The  hot  wave  began  on  the  6th,  with  a  maximum  temperature  at 
Baltimore  of  97°;  from  the  7th  to  the  12th  inclusive  the  afternoon  heat 
reached  99°  or  100°  each  day;  from  the  13th  to  the  19th  the  daily  maxi- 
mum ranged  between  90°  and  94°.  Fortunately  the  relative  humidity 
was  comparatively  low,  averaging  but  65  per  cent,  the  normal  value  being 
70  per  cent.  A  comparatively  cool  period  of  four  days  followed,  with 
heavy  showers.  The  temperature  rose  again  on  the  24th  to  87°,  and 
ranged  between  88°  and  96°  to  the  close  of  the  month.  While  the  tem- 
perature averaged  6°  less  daily  during  the  latter  period  than  from  the 
6t]i  to  the  19th,  the  relative  humidity  rose  from  65  per  cent  to  81  per 
cent.  To  add  to  the  discomfort  of  heat  and  humidity,  the  air  movement 
was  extremely  light.  The  total  wind  movement  over  Baltimore  during 
the  month  averaged  but  108  miles  per  day;  this  is  equivalent  to  an  aver- 
age of  4.5  miles  per  hour.  Such  conditions  following  closely  upon  the 
long-continued  hot  weather  of  July  and  the  first  half  of  AugTist  brought 
intense  suffering  to  man  and  beast. 

Comparing  the  hot  period  of  this  month  with  earlier  notable  hot  spells 
since  1871  we  have  the  following: 

Length  of  Period,  Averag-e  ^^aximum. 

August,   1872 12    days  93° 

August,   1888 10   days  92° 

August,   1896 10   days  94° 

August,   1900 17    days  95° 

A  particularly  uncomfortable  feature  of  the  hot  spell  was  ihe  high 
night  temperature.  During  four  successive  nights  the  minimum  tem- 
perature ranged  from  80°  to  82°.  At  no  other  time  in  the  preceding  30 
years  has  the  night  minimum  exceeded  78°.  The  normal  temperature  for 
the  month  of  August  at  Baltimore  is  75°.  During  August,  1900,  the  mean 
temperature  was  80°;  this  value  was  equalled  but  once,  namely,  in  1872. 

The  abnormally  warm  weather  of  August  was  not  confined  to  narrow 
limits.  During  the  first  week  the  temperature  was  above  normal  from 
the  Eocky  Mountains  eastward  to  the  Lower  Lakes  and  the  Appalachian 


MAKYLAXD    WEATHER   SERVICE  459 

Mountains.  In  South  Dakota  the  daily  excess  was  13°  above  the  normal 
value.  During  the  second  week  the  warm  area  extended  eastward  to  the 
Atlantic  coast,  and  the  areas  of  maximum  excess  were  transferred  east- 
ward to  Michigan  and  to  the  region  including  Philadelphia,  Baltimore, 
and  Washington,  D.  C.  The  temperature  continued  abnormally  high  dur- 
ing the  third  and  fourth  weeks,  but  the  maximum  daily  excess  fell  from 
12°  to  9°. 

The  high  temperatures  have  frequently  been  attributed  in  the  daily 
press  to  a  greater  solar  activity  as  shown  by  the  increasing  number  of 
spots  upon  the  sun's  disk.  A  less  remote  and  more  plausible  explanation 
may  be  found  in  the  unusual  distribution  of  atmospheric  pressure  during 
the  hot  spell.  There  is  a  type  of  pressure  distribution  which  always 
brings  warm  weather  to  the  Middle  Atlantic  states.  When  the  barometer 
is  high  over  the  South  Atlantic  states,  or  just  off  the  coast,  while  it  is 
relatively  low  over  an  extensive  area  to  westward  and  northward,  the 
winds  over  the  Middle  Atlantic  states  are  generally  from  a  southerly 
direction,  and  light  in  force,  while  the  skies  are  clear.  Near  the  center 
of  high  pressure,  moreover,  the  air  descends  from  higher  levels  and  is 
warmed  by  compression  in  descending.  These  conditions,  all  favorable 
to  the  production  of  high  temperatures,  were  present  in  a  marked  degree 
during  the  period  of  hot  weather  in  July  and  August.  Clear  skies  favored 
the  rapid  warming  up  of  the  surface  of  the  earth  and  the  adjacent  layers 
of  air  during  the  day;  and  the  frequent  calms  and  the  prevailing  light 
winds — the  average  for  the  entire  period  of  the  hot  spell  being  but  4.5 
miles  per  hour  at  the  Baltimore  station — prevented  the  rapid  exchange 
of  temperatures  between  adjacent  regions,  or  between  upper  and  lower 
layers  of  the  atmosphere.  As  a  result  the  air  near  the  surface  of  the 
earth  was  excessively  heated.  At  the  high  level  stations  of  Western 
Maryland  the  temperatures  were  comparatively  moderate.  The  maxi- 
mum for  the  month  of  August  was  but  89°  at  Deer  Park,  and  91°  at 
Orantsville. 

General  Weather  Conditions  During  the  Hot  Spell  of  1900. 
The  distribution  of  pressure  and  general  weather  conditions  at  the 
beginning  of  the  August  hot  spell  are  shown  in  the  cliart  for  August 


460 


THE    CLIMATE    OF   BALTIMORE 


6,  1900.  An  extensive  area  of  high  pressure  which  had  drifted  slowly 
across  the  Lake  region  moved  southward,  the  center  being  over  the  Middle 
Atlantic  states  on  the  5th.  The  center  of  the  high  area  remained  for 
nearly  two  weeks  in  approximately  the  same  position.  Clear  skies  and 
light  southerly  winds  were  the  prevailing  conditions  in  the  Middle 
Atlantic  states.     Occasionallv  the  center  of  the  high  area  would  be  a 


Fig.  161.— Chart  of  August  6,  1900  (during  Hot  Spell). 


but  the  atmosphere  was  drawn  from  the  same  source  to  the  south,  and 
little  further  to  the  southwest,  causing  a  northwest  wind  at  Baltimore, 
change  in  local  direction  would  bring  about  no  change  in  temperature. 
Over  the  Eastern  Eocky  Mountain  slope  the  pressure  remained  compara- 
tively low  throughout  the  heated  term.  On  the  12th  of  August  a 
trough  of  low  pressure  developed  between  the  southern  high  area  and  an 
area  of  high  pressure  over  the  Canadian  Provinces,  causing  cloudiness 
and    thunderstorms    in    the    Lake   region;    this    condition    developed    a 


MAllTLAXD    WEATHER    SERVICE  461 

depression  over  the  Lower  Lake  region  on  the  13th,  attended  by  showers 
and  thunderstorms  as  far  south  as  Maryland  and  Xorthern  Virginia. 
But  the  relief  brought  about  by  these  showers  was  only  temporary.  Local 
showers  in  connection  with  thunderstorms  also  afforded  some  relief  in 
the  Middle  Atlantic  states,  the  Ohio  and  the  Missouri  valleys  on  the 
16th,  but  the  temperatures  soon  regained  their  intensity.  On  the  19th 
an  area  of  high  pressure  developed  to  the  north  of  the  Lake  region  while 
the  South  Atlantic  states  high  area  drifted  to  the  southwest  and  gradually 
dissipated.  In  the  meantime  a  trough  of  low  pressure  developed  between 
the  two  high  areas  in  the  Middle  Atlantic  states,  bringing  clouds  and 
rain  and  breaking  up  the  general  conditions  of  pressure  which  caused  the 
hot  spell. 

The  high  temperatures  of  a  hot  spell  are  generally  first  experienced 
in  the  Missouri  and  Mississippi  valleys;  the  distribution  of  pressure  as 
shown  in  the  chart  for  August  5  indicates  very  favorable  conditions  for 
a  strong  drift  of  warm  southerly  winds  into  these  valleys.  The  area  of 
excessive  temperatures  then  moves  eastward  toward  the  Atlantic  sea- 
board. This  is  clearly  shown  in  the  accompanying  charts  which  outline 
the  areas  over  which  the  temperatures  were  in  excess  or  deficiency  of  their 
normal  values  for  each  week  from  July  23,  1900,  to  September  2-1,  1900. 
(See  Plate  XXIII.)  Beginning  with  the  week  ending  July  30,  we  find 
the  line  of  no  departure  from  the  normal  temperature  for  the  week 
passing  through  Baltimore,  and  that  over  practically  the  entire  central 
portion  of  the  country  the  temperatures  were  from  1°  to  3°  below  their 
normal  values.  In  tlie  accompanying  charts,  while  the  line  of  zero 
change  again  passes  through  Baltimore,  the  "  hot  wave "'  had  already 
been  well  established  in  the  Upper  Missouri  Valley  where  the  daily 
average  temperatures  were  9°  to  12°  above  their  seasonal  values.  By 
the  close  of  the  following  week  the  area  of  greatest  excess  of  temperature 
above  the  normal  was  transferred  to  ^Maryland,  Pennsylvania,  and  Vir- 
ginia with  a  departure  of  12^  per  day  above  the  normal  for  the  season, 
while  the  temperatures  had  somewhat  abated  in  Missouri  and  Mississippi 
valleys.  The  following  charts  show  that  the  unseasonable  temperatures 
continued  without  interruption  over  practically  all  of  the  country  east  of 


462  THE    CLIMATE    OF   BALTIMORE 

the  Kocky  Mountains  until  the  middle  of  September,  the  week  ending 
September  24  showing  the  first  appearance  of  temperatures  below  the 
seasonal  averages  in  the  northern  half  of  the  country,  while  they  still 
continued  high  south  of  the  Ohio  Eiver. 

It  is  interesting  to  note  in  these  charts  that  the  area  of  the  hot  wave 
embraced  practically  all  of  the  country  east  of  the  Eocky  Mountains, 
and  that  west  of  the  mountain  range  the  temperatures  were  below  their 
seasonal  averages.  This  is  generally  true  of  our  hot  waves,  the  Kocky 
Mountains  forming  a  natural  boundary  between  areas  of  excessive  and 
deficient  temperature. 

THE    SUMMER    OF    1901. 

During  the  latter  part  of  June  and  the  first  week  of  July,  1901,  a 
heated  term  of  even  greater  intensity  than  that  of  August  of  the  pre- 
ceding year  occurred,  although  it  was  fortunately  of  shorter  duration. 
Afternoon  temperatures  exceeding  90°  at  Baltimore  began  on  June 
26  with  an  area  of  high  pressure  centered  over  the  Middle  Atlantic  state? 
and  a  barometric  depression  over  the  Upper  Missouri  Valley.  The 
temperature  rose  steadily  until  the  first  of  July,  reaching  a  maximum 
of  103°  on  the  1st  and  2d;  from  the  3d  there  was  a  steady  fall  in  the 
mean  temperature  of  the  day  to  a  normal  condition  on  the  7th,  when  a 
thunderstorm  accompanied  by  heavy  rain  brought  on  an  abrupt  fall  of 
30°  in  the  temperature  between  4  o'clock  and  4.15  p.  m. 

The  excessive  heat  began  about  10  days  earlier  in  the  Central  West. 
During  the  weeic  ending  June  17  the  temperature  rose  to  6°  above  the 
normal  seasonal  value  in  the  Middle  Mississippi  Valley.  In  the  follow- 
ing week  the  area  of  excessive  heat  embraced  all  the  district  between  the 
Eocky  Mountains  and  the  Alleghanys,  with  a  maximum  departure  from 
the  normal  still  in  the  Middle  Mississippi  Valley.  By  July  1  the  area 
had  extended  to  the  Atlantic  coast,  while  the  heat  was  steadily  increasing 
in  intensity  in  the  Mississippi  Valley  and  the  Lake  region  to  a  daily  maxi- 
mum excess  of  12°  above  the  normal  temperatures.  During  the  follow- 
ing week  the  center  of  the  heated  area  was  transferred  eastward  to  the 
Middle  Atlantic  states  with  a  maximum  dailv  excess  of  12°  within  the 


VOLUME  2,   PLATE  XXIII. 


*»-«  •    / 


Fig.  4.— Week  ending  August  20,  1900. 


r' 


Fig.  8.— Week  ending  September  17,  1900. 


i 


FIGURES  1-9  SHOW 

TEMPERATURE  DEPARTURES  DURING  THE  HOT 
SPELL  OF  1900. 

Black  lines  show  temperature  departures  below  normal  during  hot  spell 

of    IQOO. 

Red  lines  show  temperature  departures  above  normal  during  the  hot 
spell  of  1900. 


)0. 


I  MARYLAND  WEATHER  SERVICE. 


VOLUME  2,  PLATE  XXIII. 


Fig.  1.— Week  ending  July  30,  1900. 


Fio.  2. — Week  ending  August  6,  1900. 


FiQ.  3. — Week  ending  August  13,  1900. 


Fio.  4.— Week  ending  August  20,  1900. 


FiQ.  B.— Week  ending  August  27,  1900. 


Fio.  G. — Week  ending  September  3,  1900. 


Fio.  7. — Week  ending  September  10,  1900. 


Fio.  8. — Week  ending  September  17,  1900. 


FIGURES  1-9  SHOW 

TEMPERATURE  DEPARTURES  DURING  THE  HOT 
SPELL  OF  1900. 

Black  lines  sliow  temperature  departures  below  normal  during  hot  spell 
of  1000. 

Red  lines  show  temperature  departures  above  normal  during  the  hot 
spell  of  1900. 


Fio.  9.— Week  ending  September  24,  1900. 


Fm.  in. — Maximum  temperatures  of  .luly,  1901. 


Fro.  11. — Maximum  temperatures  of  August,  1900. 


MAEYLAND   WEATHER    SERVICE  463 

area  embracing  Baltimore,  Philadelphia,  and  New  York,  while  the  heat 
had  somewhat  moderated  in  the  Middle  Mississippi  A'alle}-.  In  the  2d 
week  in  July  the  heat  was  again  on  the  increase  in  the  Middle  Mississippi 
Valley,  with  moderating  temperatures  in  the  Middle  Atlantic  states  and 
the  Ohio  Valley.  The  New  England  states  experienced  but  little  of  the 
excessive  heat  of  this  period.  In  the  Middle  West  during  the  second  week 
of  July  maximum  temperatures  of  102°  to  104°  were  of  frequent  occur- 
rence, establishing  new  records  for  excessive  heat  in  a  number  of  localities. 
In  Baltimore  the  hot  spell  continued  about  10  days,  while  the  highest  tem- 
perature, 103°  on  the  1st  and  2d  of  July,  was  within  1°  of  the  highest 
ever  recorded  at  Baltimore. 

THE  HOT  PERIODS  OF  AUGUST^  1900,  AND  JULY,  1901,  CO.MPARED. 

The  two  periods  of  excessive  heat  described  above  were  the  most  intense 
noted  in  the  official  records  of  the  Weather  Bureau  since  the  establishment 
of  the  Baltimore  office  of  the  National  Bureau.  While  there  were  many 
characteristics  in  common,  the  two  periods  showed  a  marked  difference 
in  their  effects  upon  the  residents  of  Baltimore.  The  death  rate  is 
always  increased  during  a  well  marked  hot  spell  in  the  large  cities  of  the 
country.  It  is  a  difficult  matter,  however,  to  determine  the  immediate 
cause  of  the  increased  rate.  It  can  not  in  general  be  attributed  alone  to 
increase  in  temperature  of  these  hot  spells,  though  this  is  probably  the 
dominant  factor.  The  humidity  doubtless  plays  an  important  part  in 
increasing  the  number  of  deaths.  Perhaps,  also,  the  weather  condi- 
tions of  the  preceding  weeks  must  be  taken  into  account.  The  hot  spell 
of  August,  1900,  covered  a  slightly  longer  period  than  that  of  June- 
July,  1901;  the  temperatures  also  averaged  somewhat  higher-;  the  wind 
movement  was  approximately  the  same.  There  was  an  astonishing  dif- 
ference, however,  in  the  number  of  deaths  reported  by  the  Baltimore 
Health  Department  as  due  directly  to  heat. 

During  the  1900  period  there  were  32  deaths  due  to  heat  prostration; 
in  the  June  and  July  period  of  1901  there  were  twice  as  many — namely, 
64.  The  only  marked  difl'^rence  between  weather  conditions  noted  was 
the  difference  in  humidity — in  1901  the  average  daily  relative  humidity 


464 


THE    CLIMATE    OF   BALTIMORE 


was  66  per  cent  of  saturation,  while  in  1900  it  was  57  per  cent.  The 
August,  1900,  period  was  preceded  by  excessive  temperatures  in  May 
and  June,  though  of  short  duration,  and  by  an  exceptionally  long  and 


1                                                     AUG             1900 

5                                     }0                                    15                                    20 

JUNE                                           JULY                 1 901 
25                                  30                                     5 

lO 

o' 

/ 

- 

\ 

/ 

/ 

> 

\ 

/ 

/ 

\ 

1   1    1 

M  AXI  MUM 

^^  1    X. 

\ 

J 

\ 

M 

V,X 

mum; 

/ 

-^ 

/ 

\ 

h  S*"*" 

1 

/ 

\ 

a-. 

\ 

/- 

\ 

/ 

\ 

M 

IN 

Ml 

JM 

/ 

^ 

villi 
\    MINIMUM 

\  1  /r\  \ 

\ 

/ 

\ 

V- 

y 

\ 

/ 

\ 

N 

\ 

1 

n° 

^ 

/ 

\ 

/ 

N 

ORMA 

L 

57° 

\ 

1      1      1      1 

NORMAL.   67- 

\ 

V  ^ 

1 

\ 

X. 

■^ 

/ 

\ 

6 

o° 

^ 

V'' 

^^~^ 

// 

/ 

/ 

\ 

^=r-^ 

// 

^ 

f«-%J 

"     ■ — -^ 

/ 

^ 

^ 

-\ 

^ 

Fig.  1G2.— Temperature  during  Hot  Spells  of  1900  and  1901. 


intense,  though  interrupted,  spell  in  July,  which  may  have  increased 
the  powers  of  resistance  and  enabled  the  residents  of  Baltimore  to  with- 
stand the  debilitating  effects  of  an  additional  period  after  the  interval 
of  two  weeks  of   moderate  summer  weather  between  the  2 2d  of  Jidy 


MARYLAND   WEATHER    SERVICE 


465 


and  6th  of  August.  In  the  case  of  the  hot  period  of  June  2  6- July  7, 
1901,  there  were  practically  no  excessively  hot  days  in  the  months  of 
May  and  the  first  three  weeks  of  June;  the  city  was  overwhelmed  with- 
out previous  preparation,  by  one  of  the  most  intense  heated  terms  exper- 
ienced  in   Baltimore. 

A  comparison  of  the  chief  climatic  features  of  the  two  periods  by 
means  of  statistics  and  diagrams  will  enable  the  reader  to  understand 
more  fully  the  points  of  difference  and  similarity. 


THE  HOT  PERIOD  OP  AUGUST,  1900. 


Date 


Aug. 


Max. 
Temp. 

Min. 
Temp. 

Relative 
Humid- 
ity. 

Wind 
Direc- 
tion. 

Hourly 

Wind 

V^elocity. 

Cloud- 
iness. 

Rain- 
fall. 

Thun- 
der 
Storms 

(Degrees  Fahr.) 

(Per  cent.) 

(Miles.) 

* 

6 

97 

67 

64 

SW 

2.9 

Clear 

7 

100 

76 

58 

W 

4.9 

Clear 

8 

99  . 

80 

52 

NW 

5.5 

Pt.  cldy 

9 

100 

81 

54 

W 

4.0 

Pt.  cldy 

10 

100 

80 

50 

Var. 

5.1 

Clear 

11 

100 

82 

44 

W 

4.7 

Clear 

12 

99 

73 

70 

'w 

6.1 

Clear 

0.14 

* 

13 

92 

72 

63 

SW 

3.7 

Pt.  cldy 

0.03 

14 

94 

76 

60 

SE 

4.0 

Pt.  cldy 

15 

91 

76 

72 

S 

4.3 

Pt.  cldy 

T 

* 

16 

92 

71 

74 

w 

4.0 

Pt.  cldy 

0.36 

* 

17 

91 

75 

73 

N 

4.5 

Pt.  cldy 

18 

92 

72 

75 

S 

3.2 

Pt.  cldy 

Total 

ige 

95.0 

75.5 

62 

w 

4.4 

Pt.  cldy 

0.53 

THE   HOT   PERIOD  OF  JUNE-JULY,    1001. 


June  26 

92 

70 

58 

SW 

3.7 

Pt.  cldy 

"       27 

92 

73 

60 

SW 

5.5 

Pt.  cldy 

"       28 

93 

73 

70 

SE 

4.0 

Pt.  cldy 

"       29 

96 

74 

68 

SW 

5.2 

Clear 

"       30 

99 

77 

58 

W 

3.1 

Clear 

July    1 

103 

80 

60 

SW 

3.6 

Pt.  cldy 

2 

103 

80 

65 

Var. 

4.5 

Pt.  cldy 

"         3 

97 

74 

60 

SW 

4.9 

Pt.  cldy 

T 

4 

96 

77 

66 

w 

4.3 

Pt.  cldy 

T 

5 

94 

76 

61 

SW 

5.9 

Pt.  cldy 

6 

96 

69 

76 

W 

6.0 

Pt.  cldy 

0.65 

7 

90 

66 

83 

SW 

5.8 

Pt.  cldy 

0.50 
Total 

Average 

95.9 

74.1 

65 

SW 

4.7 

Pt.  cldy 

1.15 

466 


THE    CLIMATE    OF   BALTIMORE 


THE    ANNUAL    DISTRIBUTION    OF    DAYS    WITH    A    MAXIMUM    TEMPERATURE 


Tear.       Apr.     May. 


OF  90°   OR  ABOVE. 

(Baltimore,  Md.,  1871-1907.) 
June.  July.  Aug.    Sept.     Oct.    Annual.  Absolute  Maximum. 


1871 

2 

5 

2 

9 

92, 

July   16. 

1872 

5 

10 

12 

2 

..        29 

97, 

July   2. 

1873 

2 

15 

4 

2 

..        23 

96, 

July    3. 

1874 

, . 

9 

7 

3 

1 

20 

98, 

June  9. 

1875 

7 

7 

2 

16 

97, 

June  27. 

1876 

5 

18 

2 

..        25 

99, 

July    9. 

1877 

2 

4 

8 

3 

17 

95, 

June  26. 

1878 

1 

16 

4 

21 

98, 

July    18. 

1879 

1 

4 

10 

5 

..        20 

99, 

July    16. 

1880 

7 

9 

10 

2 

3 

..        31 

99, 

July    13. 

1881 

3 

3 

11 

8 

6 

31 

101, 

Sept.  7. 

1882 

6 

8 

1 

15 

97, 

June  25. 

1883 

1 

7 

2 

10 

96, 

July    22. 

1884 

4 

3 

3 

3 

..        13 

95, 

July    24. 

1885 

4 

15 

3 

22 

99, 

July    21. 

1886 

1887 

2 

4 
10 

4 
3 

2 

10 
15 

92, 
102, 

rJuly    7, 

jAug.    27 

July    18. 

1888           1 

8 

5 

10 

24 

96, 

Aug.   16. 

1889 

2 

5 

1 

10 

93, 

July    9. 

1890 

4 

8 

2 

14 

98, 

July    8, 

1891 

5 

5 

1 

11 

94, 

^  June  16, 
)  Aug.   10,  11 

1892 

6 

10 

o 

19 

99, 

July   26. 

1893 

5 

9 

2 

16 

98, 

June  20. 

1894 

8 

11 

2 

2 

23 

98, 

June  24. 

1895 

2 

5 

5 

10 

7 

29 

97, 

June  1,  3. 

189G           2 

5 

3 

10 

10 

3 

33 

98, 

Aug.   7. 

1897 

2 

4 

1 

4 

1       12 

97, 

Sept.  11. 

1898 

1 

7 

10 

9 

8 

35 

104, 

July   3. 

1899 

1 

8 

8 

8 

2 

27 

98, 

June  6. 

1900 

o 

3 

15 

17 

4 

.  .       42 

100, 

(July   16,  17 
/  Aug.  7,  9,  1 

1901 

6 

18 

1 

1 

26 

103 

July   1,   2. 

1902 

1 

4 

10 

1 

1 

17 

99 

July    18. 

1903           1 

1 

10 

2 

14 

94 

Aug.    25. 

1904 

4 

6 

10 

97 

July    19. 

1905 

4 

8 

1 

13 

98 

July    18. 

1906 

2 

5 

3 

4 

2 

16 

96 

Aug.    6. 

1907 

1 

6 

3 

1 

11 

93 

July   8,  11. 

Totals       4 

31 

158 

325 

153 

57 

1     729 

Average  . . 

1 

4 

9 

4 

2 

20 

makyland  weather  service  467 

The  Cold  Summer  of  1816. 

There  are  numerous  records  in  local  annals  showing  that  the  summer 
of  1816  was  phenomenally  cold — in  fact  the  coldest  of  which  we  have  any 
authentic  records.  Systematic  instrumental  observations  did  not  begin 
in  Baltimore  until  the  year  1817  (see  pages  91-95) ;  however,  it  is  not 
a  difficult  matter  to  reduce  reliable  records  of  a  neighboring  station  to 
contemporary  conditions  in  Baltimore.  We  fortunately  have  a  very 
complete  and  trustworthy  series  of  daily  records  for  Philadelphia,  which 
go  back  to  the  year  1790. 

In  the  main,  weather  changes  in  Philadelphia  and  Baltimore  are 
synchronous,  and  similar  in  kind;  there  is,  however,  a  uniform  difference 
of  1°  to  2°  in  the  average  monthly  temperatures  of  the  two  stations 
due  to  difference  in  latitude.  By  adding  this  difference  to  the  average 
monthly  Philadelphia  temperatures  we  obtain  a  reliable  value  for  con- 
temporary Baltimore  temperatures. 

An  interesting  little  book  published  in  Philadelphia  in  1847  by  Mr. 
Charles  Peirce  and  entitled  "  A  meteorological  account  of  the  weather  in 
Philadelphia  from  1790  to  1847,^'  contains  a  valuable  record  of.  the 
general  weather  conditions  for  each  month  during  this  period  of  57 
years. 

The  following  extracts  are  made  concerning  the  character  of  the 
weather  conditions  in  1816,  with  special  reference  to  the  three  summer 
months  of  June,  July,  and  August : 

The  Year.  The  temperature  of  the  whole  year  was  only  49°;  it  being  the 
coldest  year  we  have  on  record.  Although  there  was  no  uncommonly  cold 
weather  during  the  three  winter  months,  yet  there  was  ice  during  every 
month  in  the  year,  not  excepting  June,  July,  and  August.  There  was  scarcely 
a  vegetable  came  to  perfection  north  and  east  of  the  Potomac.  The  cold 
weather  during  summer,  not  only  extended  through  America,  but  throughout 
Europe.  It  was  also  the  coldest  summer  ever  known  in  the  West  Indies  and 
in  Africa. 

June,  181G.  The  medium  temperature  of  the  month  was  only  64°,  and  it 
was  the  coldest  month  of  June  we  ever  remember;  there  were  not  only  severe 
frosts  on  several  mornings,  but  on  one  morning  there  was  said  to  be  ice. 
Every  green  herb  was  killed,  and  vegetables  of  every  description  very  much 
injured.  All  kinds  of  fruit  had  been  previously  destroyed,  as  not  a  month 
had   passed   without  producing  ice.     From   6   to   10   inches   of   snow   fell   in 


468  THE    CLIMATE    OF   BALTIMORE 

various  parts  of  Vermont;  3  inches  in  the  interior  of  New  York;  and  several 
inches  in  the  interior  of  New  Hampshire  and  Maine. 

July,  1816.  The  medium  or  average  temperature  of  this  month  was  only 
68°,  and  it  was  a  month  of  melancholy  forebodings,  as  during  every  previous 
month  since  the  year  commenced,  there  were  not  only  heavy  frosts,  but  ice, 
so  that  very  few  vegetables  came  to  perfection.  It  seemed  as  if  the  sun  had 
lost  his  warm  and  cheering  influences.  One  frosty  night  was  succeeded  by 
another,  and  thin  ice  formed  in  many  exposed  situations  in  the  country.  On 
the  morning  of  the  5th  there  was  ice  as  thick  as  window  glass  in  Pennsyl- 
vania, New  York,  and  through  New  England.  Indian  corn  was  chilled  and 
withered,  and  the  grass  was  so  much  killed  by  repeated  frosts,  that  grazing 
cattle  would  scarcely  eat  it.  Northerly  winds  prevailed  a  great  part  of  the 
month;  and  when  the  wind  changed  to  the  west,  and  produced  a  pleasant 
day,  it  was  a  subject  of  congratulation  by  all.  Very  little  rain  fell  during 
the  month. 

August,  1816.  The  medium  temperature  of  this  month  was  only  66°,  and 
such  a  cheerless,  desponding,  melancholy  summer  month,  the  oldest  inhabi- 
tant never,  perhaps,  experienced.  This  poor  month  entered  upon  its  duties 
so  perfectly  chilled,  as  to  be  unable  to  raise  a  warm,  foggy  morning,  or 
cheerful  sunny  day.  It  commenced  with  a  cold  northeast  rain  storm,  and 
when  it  cleared  the  atmosphere  was  so  chilled  as  to  produce  ice  in  many 
places  half  an  inch  thick.  It  froze  the  Indian  corn,  which  was  in  the  milk, 
so  hard,  that  it  rotted  up  on  the  stock,  and  farmers  mowed  it  down  and  dried 
it  for  cattle  fodder.  Every  green  thing  was  destroyed,  not  only  in  this 
country,  but  in  Europe.  Newspapers  received  from  England  said:  "It  will 
be  remembered  by  the  present  generation,  that  the  year  1816  was  a  year  in 
which  there  was  no  summer."  Indian  corn,  raised  in  Pennsylvania  in  1815, 
sold  (for  seed  to  plant  in  the  spring  of  1817)  for  four  dollars  per  bushel  in 
many  places. 

The  departures  of  the  year  1816  from  the  normal  summer  temperature 
are  compared  with  the  departures  for  some  of  the  coolest  summers  on 
record  in  Baltimore  in  the  following  table : 

DEPARTURES   FROM   THE    NORMAL  TEMPERATURE. 


June. 

July. 

August. 

Season 

1816 

—8° 

—8° 

—8° 

—8.0° 

1836 

—5° 

—3° 

—5° 

—4.3° 

1846 

—4° 

—3° 

—1° 

—2.7° 

1886 

—3° 

—3° 

—2° 

—2.7° 

1891 

—1° 

—6° 

—2° 

—3.0° 

1903 

—6° 

—1° 

—3° 

—3.3° 

The  mean  daily  temperature  for  each  month  of  the  summer  of  1816 
fell  decidedly  below  that  of  any  summer  month  during  the  period  of 
observations — from  1790  to  1906. 


MARYLAND   WEATHER    SERVICE  469 

DlSTRIBUTIOX    OF    PRESSURE    DURIXG    THE    CoOL    JUXE    OF    1903. 

In  the  normal  distribution  of  pressure  during  the  summer  months 
the  western  edge  of  the  Atlantic  high  area  extends  to  the  South  Atlantic 
states  while  the  barometer  over  the  Central  states  is  comparativeh'  low. 
Hence  the  atmosphere  which  flows  over  the  ^liddle  Atlantic  states  comes 
from  the  warm  southeast.  In  June  of,  1903  the  barometer  was  com- 
paratively high  over  the  Xorth-Central  states,  with  a  maximum  in  the 
Upper  Mississippi  and  in  the  Missouri  valleys,  and  relatively  low  in 
the  Lower  Lake  region  and  the  Atlantic  coast  states.  During  the  same 
period  the  western  portion  of  the  Atlantic  high  area  was  found  far  north- 
ward toward  the  Gulf  of  St.  Lawrence,  causing  cool  easterly  in  place 
of  the  usual  warm  southerly  winds  to  blow  over  the  Middle  Atlantic 
states ;  in  addition  an  unusual  flow  of  cool  air  was  derived  from  the  high 
area  to  the  northwest  over  the  central  portion  of  the  North  American 
continent.  The  effect  of  this  abnormal  distribution  of  pressure  is 
reflected  in  the  temperature  departures  recorded  in  the  following  table: 

COOL  JUNE  OF  1903. 
Districts.  Normal  Temp.       Departure. 

New  England  States 57.8°  —5.4° 

Middle  Atlantic  States 65.7°  —5.2° 

South  Atlantic  States 73.4°  —3.2° 

Gulf  States    74.5°  —4.5° 

Ohio  Valley  and  Tennessee 68.4°  — 5.6° 

Lake  Region     60.8°  —3.8° 

At  Baltimore  the  month  was  cool,  wet,  and  cloudy.  The  afternoon 
temperatures  were  high  on  a  number  of  days,  but  the  warm  periods 
were  of  brief  duration.  Light  frosts  occurred  in  the  mountains  at  the 
beginning  of  the  month.  The  rainfall  was  considerably  in  excess  of  the 
normal  seasonal  amounts.  The  mean  temperature  of  the  month  was 
67.0°,  5.8°  below  the  average  value  for  a  period  of  86  years.  This  large 
departure  from  the  average  June  temperature  in  Baltimore  marks  the 
month  of  June,  1903,  as  the  coldest  since  the  beginnipg  of  systematic 
instriitnciital  observations  in  1817.     (See  PL  XX1\.) 


470  the  climate  of  baltimore 

Distribution  of  Pressure  During  the  Normal  June  of  1902. 

The  temperatures  during  the  month  of  June,  1903,  were  very  near 
the  normal  for  a  long  series  of  years;  they  were  remarkably  uniform, 
the  month  being  without  marked  departures  from  the  seasonal  average 
either  above  or  below  the  normal.  The  distribution  of  pressure  was  in 
marked  contrast  with  that  of  the  cool  June  of  1903,  described  in  the 
preceding  paragraphs.  The  pressure  was  highest  over  the  South  Atlantic 
states  and  low  over  the  St.  Lawrence  Valley  and  the  extreme  Southwest. 
The  pressure  distribution  was  such  as  to  give  prevailing  southwest  winds 
over  the  Middle  Atlantic  states.     (See  PI.  XXIV.) 

While  the  immediate  cause  of  departures  from  the  normal  seasonal 
temperatures  over  wide  areas  may  be  traced  back  to  abnormal  distribu- 
tion of  pressure,  it  is  not  so  easy  to  find  a  cause  for  these  abnormal 
movements  in  the  positions  of  the  so-called  permanent  areas  of  high  and 
low  pressure.  This  slow  shifting  about  of  the  large  areas  of  high  and 
low  barometric  pressure  is  sufficient  cause  for  the  greatest  observed  depar- 
tures from  the  seasonal  temperature  of  a  given  locality,  without  calling 

* 

in  the  airl  of  extra-terrestrial  influences,  such  as  the  moon,  the  planets, 
or  sun-spots. 

The  Variability  of  Summer  Temperatures. 

While  marked  temperature  changes  from  day  to  day  during  the  summer 
months  are  not  as  frequent  or  as  large  as  they  are  during  the  winter  and 
spring  seasons,  there  is  still  considerable  variability  due  to  the  different 
types  of  pressure  distribution.  With  a  pronounced  area  of  high  pres- 
sure over  the  Upper  Mississippi  Valley  and  the  Lake  region  the  tem- 
peratures in  the  Middle  Atlantic  states  fall  below  the  seasonal  average; 
with  a  well  developed  high  area  over  the  South  Atlantic  states,  or  just 
off  the  coast  to  the  southeast,  the  temperatures  over  the  Middle  Atlantic 
and  the  Central  states  rise  above  the  normal.  These  two  types  of  pres- 
sure distribution,  with  resulting  departures  from  the  normal  seasonal 
temperatures,  are  well  defined  in  the  weather  maps  of  July  1,  1885,  and 
July  1,  1901.     On  the  1st  of  July,  1885,  an  area  of  high  pressure  cov- 


VOLUME  2,  PLATE  XXIV. 


^■ 


V. 


/ 


Fig.  10.— Cold  October,  1905  (—4°). 


Fig.  11.— Normal  October,  1894  (— 0°.l). 


Fig.  12— Warm  October.  1900   (-f  4°.5). 
D  States, 

BObars,  or  lines  of  equal   pressure 


/ 


MARVLAND  WEATHER  SERVICE 


E  2,  PLATE  XXIV. 


Fio.  3.— Warm  December  of  1889  (-f  8°.0). 


Pio.  6.— Warm  March  of  1898  (+G°.5). 


Fia.  9.— Warm  June  of  1899   (+2°) 


Pio.  12— Warm  October.  1900   (+4''.6). 


DiSTRIBDTION  OF  PrKSSURE,  WiNDS  AND  TEMPERATURE  DURING  NORMAL,  (^OLD   AND    WaRM    SEASONS    IN   THE   UNITED  STATES. 

Black   lines   are    isotherms,   or   lines   of  equal    temperature.  Red  lines  are  isobars,  or  lines  of  equal  pressur 

Arrows  fly   with   the  wind. 


MARYLAND   WEATHER    SERVICE 


471 


FiG.  1G3.— The  Cold  July  1,  18S5. 


31 


Fic.  1G4.— The  Wiirni  July  ],  1901. 


472  THE    CLIMATE    OF   BALTIMORE 

ered  most  of  the  country  east  of  the  Kocky  Mountains,  excepting  the 
New  England  states,  the  barometer  being  highest  over  the  Middle  Mis- 
sissippi Yalley.  This  distribution  of  pressure  caused  a  steady  flow  of 
cool  northwest  winds  over  the  Middle  Atlantic  states.  The  tempera- 
tures for  the  day  were  abnormally  low.  The  early  morning  minimum 
at  Baltimore  was  56°,  the  lowest  minimum  recorded  on  the  first  day 
of  July  in  a  period  of  36  years.  The  distribution  of  pressure  noted  above 
is  typical  for  periods  of  cool  weather  in  all  seasons  of  the  year. 

TEMPERATURES  ON  JULY  1,  1885. 
(A  cool  day  with  high  barometer  in  Northwest.) 

7  a.m.  o  p.  m.  11p.m.  Max.  Min. 

62  76  68  79  56 

Mean  67.5° 

On  the  other  hand  the  warm  weather  type  is  represented  by  the  dis- 
tribution of  pressure  seen  in  the  weath?r  map  showin.u-  conditions  on  the 
morning  of  July  1,  1901. 

In  this  type  the'  high  area  covers  the  South  Atlantic  states.  With 
such  a  distribution  of  pressure  the  winds  in  the  Middle  Atlantic  sta'tes 
are  light  and  prevailingly  from  the  south  or  southwest  and  abnormally 
warm.  The  minimum  temperature  on  this  day  at  Baltimore  was  80°, 
while  the  afternoon  maximum  reached  103°,  the  liighest  recorded  in 
Baltimore  upon  the  first  day  of  July. 

TEMPERATURES  ON  JULY  1,   1901. 
(A  warm  day  with  high  barometer  in  the  Southeast.) 


Hours. 

2 

4 

6                   8 

10 

13 

A.  M. 

82 

81 

82                 88 

96 

103    Noon 

P.M. 

103 

99 

96                 91 
Mean  90.9° 

88 

87    Midnight 

THE    WEATHER    OF    JULY    4. 

The  variability  of  weather  conditions  may  be  illustrated  in  another 
way,  by  charting  the  various  climatic  factors  for  a  given  typical  summer 
day  during  a  long  series  of  years.  Thus  in  the  accompanying  diagram 
we  have  noted  the  maximum,  the  mean,  and  the  minimum  temperatures, 
the  barometric  pressure,  the  amount  of  cloudiness,  the  prevailing  wind 
direction  and  the  amount  of  rainfall  recorded  upon  each  fourth  of  July 


MARYLAND   WEATHER   SERVICE 


473 


TOE   WEATHER  OF  JULY  4. 


Year. 
1871 

Max. 
Temp. 

82 

Miu. 
Temp. 
(Degrees 
71 

Mean 
Temp. 
Fahr.) 
76 

Character 
of  Day. 

Pt.  cldy 

Wind 
Direction. 

E&SW 

Daily  Wind 
Movement. 
(Miles) 
120 

Precip- 
itation. 
(Inches) 
0.03 

1872 

93 

78 

86 

Pt.  cldy 

SW 

117 

0.05 

1873 

92 

76 

84 

Cloudy 

W 

168 

1874 

92 

67 

80 

Cloudy 

S&W 

100 

1.14 

1875 

83 

69 

76 

Cloudy 

SE 

137 

1876 

95 

73 

84 

Pt.  cldy 

SW 

103 

0.22 

1877 

86 

70 

78 

Pt.  cldy 

N 

185 

1878 

92 

73 

82 

Clear 

SE 

127 

1879 

98 

74 

86 

Pt.  cldy 

SW&W 

170 

1880 

87 

66 

76 

Clear 

N 

145 

1881 

93 

74 

84 

Cloudy 

NW 

137 

1882 

71 

61 

66 

Cloudy 

NE 

178 

1.09 

1883 

94 

74 

84 

Clear 

SW 

207 

1884 

85 

73 

79 

Cloudy 

E-SE-S 

115 

0.04 

1885 

88 

65 

77 

Clear 

N&NW 

85 

1886 

86 

66 

76 

Pt.  cldy 

N-E-S 

70 

1887 

86 

72 

79 

Cloudy 

SE 

212 

1888 

85 

64 

74 

Pt.  cldy 

S 

143    . 

1889 

84 

74 

79 

Cloudy 

SW 

143 

0.36 

1890 

91 

71 

81 

Pt.  cldy 

SW&NW 

77 

1891 

79 

59 

69 

Pt.  cldy 

W&NW 

324 

0.08 

1892 

75 

61 

68 

Pt.  cldy 

W&NW 

191 

1893 

84 

64 

74 

Clear 

NW 

155 

0.01 

1894 

86 

72 

79 

Pt.  cldy 

W 

243 

1895 

76 

64 

70 

Cloudy 

NW 

169 

0.11 

1896 

88 

70 

79 

Cloudy 

SW 

177 

1897 

86 

72 

79 

Pt.  cldy 

E 

138 

1898 

100 

74 

87 

Pt.  cldy 

SW 

102 

0.07 

1889 

90 

68 

79 

Pt.  cldy 

S 

115 

1900 

97 

77 

87 

Pt.  cldy 

SW&W 

114 

1901 

96 

77 

86 

Pt.  cldy 

w 

103 

1902 

86 

70 

78 

Cloudy 

s 

117 

0.17 

1903 

87 

73 

80 

Cloudy 

SE 

123 

0.08 

1904 

88 

61 

74 

Clear 

SW 

156 

1905 

83 

69 

76 

Pt.  cldy 

SE 

204 

1906 

85 

73 

79 

Pt.  cldy 

SW&W 

217 

0.02 

1907 

78 

59 

68 

Clear 

N&E 

121 

Average 

87 

70 

78 

150 

0.25 

O 

o 

O 
05 

O 

00 

o 

o 

«5 

b 

O 

o 

O 

o 

-  i  1 

j  1 

^<^^^     i 

r- 

!?^   i 

^ 

ly" 

y\ 

: 

\i 

;v7:^    r 

Hi 

J    '■   ■^ 

/: 

<   T    ; 

o 
u 

\ 

K 

K 

V  : 

i 

3;  ^Y    ■ 

^S 

i 

i 

/ 

; 

^  : 

i 

! 

cc    JT    : 

5 

! 

—> 

-1 : 

V 

:\ 

i 

y 

*-^  VV  : 

^    V 

1 

: 

ii 

i 

3  1 

■\ 

1 

A 

CD 
cr: 

^-»  V^/  ' 

j 

"^ 

2i 

>^ 

H 

2 

S'' 

y 

R 

A 

\ 

' 

■      1     '■ 

■ 

! 

^  < 

\ 

: 

: 

■(_)■  Y' 

^ 

i 

SI 

\ 

\i 

'■ 

^ 

.\ 

u 

;\ 

;9i  .  ; 

o 

/^ 

x' 

/^ 

\ 

'n  ' 

*  ■ 

£ 

V 

:/ 

i 

f 

i^:^ 

1 

1 

\ 

\i 

^rS^ 

/< 

A 

1 

iV' 

' 

i 

I  < 

\i 

'n  ij 

^ 

|: 

] 

\\ 

O  '^ 

1 

I 

i 

/ 

i 

4' 

/ 

\ 

:[ 

\i 

; 

iAi, 

f 

—^ 

;;^ 

$^ 

^> 

\\ 

J 

rih  ^ 

\ 

-< 

^ 

^ 

A 

^rn' 

\  ■ 

o 

\\ 

\' 

u 

; 

rS'^ 

IB 

/ 

>-|- 

i/ 

/; 

:\ 

1 

iVi . 

\ 

\^ 

1 

u 

^XiJ 

/'  • 

3  ; 

v;'' 

>^\ 

'^ 

1/ 

''■l 

g 
-CH- 
OI 

Ja;» 

i^ 

lO 

-> 

s: 

A 

k  i 

: 

(XL 

*ci 

— i — 

22 

•^^ 

^ 

m 

i 

QC: 

i 

— r? 

rr- 

|r 

■q=  rly  ; 

L       ^ 

1 

i 

: 

y 

/i 

i 

1 

J-H 

00 

:u 

— i — 

i 

i 

ll 

iS- 

^ 

i 

MARYLAND   WEATHEE   SERVICE  475 

i'rom  1S71  to  1903.  We  see  that  the  lowest  temperature  recorded  on 
any  -ith  of  July  was  58°  and  that  this  occurred  in  1891;  the  highest 
temperature,  namely  100°,  is  credited  to  1898.  Between  these  extremes 
we  have  had  in  the  past  36  years  all  degrees  of  temperature  conditions. 
There  appears  to  be  no  regularity  in  the  fluctuations  in  temperature  from 
year  to  year,  although  there  are  indications  of  irregular  periods  of 
steadily  decreasing  or  increasing  temperature. 

The  barometric  pressure  has  varied  but  little  above  or  below  the  normal 
seasonal  values,  the  entire  range  being  less  than  half  an  inch. 

The  days  with  a  clear  sky  have  numbered  but  six  in  36  years;  with 
partly  cloudy  sky,  18 ;  and  with  an  overcast  sky,  12.  The  prevailing 
wind  direction  has  been  from  the  southwest.  Kain  fell  in  amounts  vary- 
ing from  a  light  sprinkle  to  heavy  showers  on  somewhat  less  than  half 
the  total  number  of  days,  making  the  rainfall  probability  for  the  4th  of 
July  less  than  50  per  cent.  Thunderstorms  were  recorded  but  five  times 
during  the  period  of  36  years  on  this  day. 

West  Indian  Hurricanes. 

Hurricanes  do  not  differ,  in  essential  features,  from  the  temperate 
region  cyclones  described  in  preceding  pages.  They  are  more  restricted 
in  area,  but  relatively  more  intense  in  energy  and  destructive  power. 
These  storms  have  their  origin  in  the  vicinity  of  the  Windward  Islands; 
they  move  toward  the  west  or  northwest  at  the  rate  of  10  to  12  miles 
per  hour — less  than  half  the  average  rate  of  temperate  region  cyclones — 
and  curve  northward  and  then  northeastward  approximately  in  the 
neighborhood  of  Florida,  as  a  rule,  following  the  Atlantic  coast,  enlarg- 
ing in  area  after  recurving  until  they  resemble  in  every  detail  the  storms 
common  to  the  higher  latitudes. 

While  these,  the  most  disastrous  of  all  storms,  have  occurred  in  all 
seasons  of  the  year,  they  are  confined  almost  entirely  to  the  months  of 
August,  September,  and  October.  The  abrupt  increase  in  their  frequency 
in  August  is  phenomenal  as  shown  in  the  following  extract  from  one  of 
the  publications  of  the  United  States  Weather  Bureau.' 

^  E.  B.  Garriott.  West  Indian  Hurricanes.  Bull.  H.,  U.  S.  Weather  Bureau. 
4to.     Washington,  D.  C,  1902. 


476  THE    CLIMATE    OF   BALTIMORE 

FREQUENCY   OF  HURIilCANES    (1878-1900). 


Jan. 

Feb. 

Mar. 

Apr. 

May. 

June. 

July. 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Vear. 

3 

0 

0 

0 

1 

3 

3 

25 

25 

32 

3 

3 

98 

Fortunately  the  path  of  the  hurricane  rarely  falls  within  the  limits 
of  the  Middle  Atlantic  states  until  it  has  lost  some  of  its  violence.  By 
the  time  it  has  reached  the  latitude  of  Baltimore  the  center  is  generally 
well  off  the  coast,  and  we  experience  only  the  ordinary  storm  winds  of  the 
western  quadrant  of  the  liurricane. 

When  there  happens  to  be  a  well  developed  area  of  high  barometric 
pressure  over  the  eastern  half  of  the  country  on  the  approach  of  a  hurri- 
cane the  storm  is  prevented  from  recurving  near  the  Florida  Peninsula 
and  moves  slowly  westward  into  the  Gulf  of  Mexico,  or  even  entirely 
across  the  Gulf,  before  recurving  northward.  Under  such  circumstances 
the  hurricane  is  apt  to  gather  in  force  in  its  journey  across  the  Gulf. 
The  storm  which  destroyed  Galveston  in  September,  1900,  was  of  this 
type. 

A  typical  storin  of  this  class  passed  over  Maryland  on  the  13th  of 
October,  1893  ;  it  is  described  in  the  following  paragraphs. 

THE  HURRICANE  OF  OCTOBER  13,   1893. 

The  first  indication  of  the  approach  of  this  storm  from  the  West 
Indies  was  contained  in  a  report  from  Saint  Thomas  on  the  5th  of 
October;  on  the  following  day  additional  information  was  received  from 
Antigua.  The  storm  advanced  slowly  westward.'  On  the  Tth  it  was 
southeast  of  Port  au  Prince,  and  on  the  8th  southeast  of  Santiago  de 
Cuba.  On  the  9th  it  had  reached  the  Bahama  Islands  and  Southern 
Florida,  and  storm  signals  were  ordered  up  along  the  Florida  and  east 
Gulf  coasts  by  the  Chief  of  the  Weather  Bureau.  By  the  evening  of 
the  10th  the  wind  had  freshened  to  a  gale  along  the  Florida  coast.  On 
the  morning  of  the  11th  the  storm  center  was  east  of  the  Bahama  Islands 
and  the  barometer  was  falling  rapidly  along  the  Atlantic  coast  as  far 
north  as  Xew  Jersey.     During  the  12th  severe  nortlieast  gales  and  heavy 

'.See.-    Lake  Storm  Bulletin.     No.  2,  1893,  U.  S.  Weather  Bureau. 


MAKYLAND   WKITHER   SERVICE  477 

rains  prevailed  along  the  coast  of  the  South  Atlantic  states  in  connec- 
tion with  a  rapidly  falling  barometer. 

On  the  morning  of  the  13th  the  storm  center  reached  the  South  Caro- 
lina coast,  the  barometer  at  Charleston  indicating  28.88  inches.  From 
this  point  the  storm  took  an  unusual  course,  moving  northward  into  the 
interior,  the  center  passing  over  the  Carolinas  and  the  Middle  Atlantic 
states.  Xortheast  storm  signals  were  ordered  for  all  stations  on  the 
Middle  Atlantic  and  Kew  England  coasts.  Special  w^arnings  were  sent 
to  all  Weather  Bureau  observers  in  the  Middle  Atlantic  states  and  Kew 
England,  and  observers  from  Southern  New  England  to  Maryland  were 
authorized  to  use  tlie  telegraph  at  their  discretion  in  distributing  these 
warnings  in  the  most  effectual  manner  possible. 

During  the  evening  of  the  13th  the  center  of  the  storm  passed  over 
Western  Maryland,  the  barometer  falling  to  28.88  inches  at  Baltimore. 
Moving  due  north  it  crossed  Pennsylvania  and  Western  N'ew  York  to  the 
north  of  Lake  Ontario  on  the  14th.  On  the  15th  the  st>orm  disappeared 
in  the  direction  of  Labrador. 

The  storm  was  attended  by  high  winds  and  heavy  rains  all  along  its 
path  across  the  United  States.  Some  of  the  records  are  quoted  below 
from  the  official  report  of  the  United  States  Weather  Bureau. 

niGII    WINDS   AND  HEAVY  RAINS   DURING    THE    STORM    OF   OCTOBER    13   AND 

14,   1893. 

(S  a.  m.  to  8  p.  m.  October  13.) 

„.    ..  Velocity.      Direc-  <i.t<itinn  V^elocity.     Direc- 

Stiition.  (Miles)  tion.  i^tation.  (Miles)        tion. 

Jacksonville,  Fla 38  SW  Harrisburg,   Pa 36  E 

Charleston,   S.   C 42  NW  Atlantic  City,  N.  J....38  SE 

Atlanta,    Ga 38  W  Philadelphia,  Pa 48  SE 

Wilmington.   N.   C 56  SE  New  York  City 30  SE 

Raleigh,   N.   C 36  E  Cleveland,   0 48  NE 

Southport.   N.   C SO  SE  Erie,  Pa 30  NE 

Washington,    D.   C 42  SE  Baltimore,   Md 38  SE 

BAINFALL. 
Station.  luches.  Station.  Inches. 

Raleigh,   N.   C 2.08  Baltimore,   Md 1.00 

Lynchburg,    Va 1.66  Pittsburg,   Pa 1.01 

Washington,    D.   C 1.82  Parkersburg,  W.   Va 2.48 


478 


THE    CLIMATE    OF   BALTIMORE 


Fig.  166.— The  Hurricane  of  October  13,  1893   (8  a.  m.), 


Fig.  167.— The  Hurricane  of  October  13,  1893   (8  p.  m.). 


MARYLAXD   WEATHER    SERVICE 


479 


(8  p.  m.  Oct.  13  to  S  a.  m.  Oct.  14.) 


Velocity. 
(Miles) 

...34 

...42 


Station. 
Charleston,    S.    C. 
Washington,   D.   C 

Baltimore,  Md 40 

Atlantic  City,  N.  J 44 

Philadelphia,  Pa 56 

Sandy  Hook,  N.  J 64 

Boston,  Mass 36 

Woods  Holl,  Mass 44 


Direc- 
tion. 

W 
SE 
SE 
SE 
SE 
SE 

E 
SE 


Station. 
Albany,  N.  Y.  . 
Oswego,   N.   Y. 
Buffalo,  N.  Y.  . 

Erie,  Pa 

Sandusky,  O.    . 


Velocity    Direc- 
(Miles)       tion. 


.48 
.60 
.60 
.36 
.36 


Detroit,  Mich 46 

Grand   Haven,   Mich... 36 
Marquette,  Mich 34 


SE 
SE 
SW 
SE 

NW 

W 

NW 

NW 


Fig.  168.— The  Hurricane  of  October  14,  1893   (8  a.  m.). 


RAINFALL. 


Station.  Inches. 

Philadelphia,   Pa 1.24 

Cleveland,    0 1.90 


station.  Inches. 

Alpena,   Mich 1.08 

Port  Huron,  Mich 1.50 


The  meteorological  conditions  as  recorded  at  the  Baltimore  Office  of 
the  Weather  Bureau  are  indicated  in  the  accompanying  diagram  and  in 
the  following  extracts  from  the  records  of  the  office : 


480  THE    CLIMATE    OF    BALTIMORE 

The  barometer  reached  its  lowest  point,  28.88  inches,  at  10  p.  m.  of  the 
13th,  then  slowly  rose  throughout  the  night  and  following  day.  Light  rain 
began  to  fall  with  a  northeast  wind  and  continued  without  interruption  until 
midnight.  The  total  amount  of  precipitation  was  1.60  inch.  The  maximum 
wind  velocity,  40  miles  per  hour  from  the  southeast,  occurred  during  the 
night  of  the  13th-14th  just  before  the  barometer  began  to  rise,  at  the  time 
of  heaviest  rainfall.  The  temperature  rose  slowly  and  steadily  from  a  min- 
imum of  56°  at  6  a.  m.  to  73°  at  midnight,  as  the  wind  gradually  veered  from 
northeast  to  southeast,  obliterating  the  usual  diurnal  fall  after  3  p.  m. 

The  following  morning  began  with  a  clear  sky  and  a  fresh  southwest  wind, 
the  storm  having  passed  northward  beyond  the  horizon  of  Baltimore. 

AUTUMN  WEATHEE. 

Although  the  months  of  June,  July,  and  August  only  are  allotted  to 
the  summer  season  in  the  division  of  our  calendar,  the  weather  of  the 
month  of  SejDtember  in  the  Middle  Atlantic  states  is  truly  summer 
weather.  The  temperatures  continue  high ;  the  mean  monthly  tempera- 
ture is  occasionally  higher  than  the  mean  monthly  value  for  June,  July, 
or  August,  while  tlie  greatest  heat  of  the  summer  has  on  at  least  two 
occasions  within  the  past  35  years  fallen  within  the  first  half  of  the 
month.  As  stated  in  an  earlier  paragraph  (page  78)  the  temperature 
falls  more  slowly  in  autumn  than  it  rises  in  the  spring  months.  There 
is  a  striking  difference  in  the  mean  temperature  of  the  equinoctial  days 
of  spring  and  fall,  the  latter  being  23°  warmer  than  the  former.  The 
wind  movement  is  less  than  that  of  any  other  month  of  the  year,  except- 
ing August,  in  spite  of  the  reputation  as  a  month  of  equinoctial  storms. 

AVERAGE  DAILY  WIND  MOVEMENT  AT  BALTIMORE    (1873-1903). 

Jan.      Feb.      Mar.      Apr.      May.      June.  July.    Aug-.      Sept.      Oct.      Nov.    Dec.    Year. 

145    103    175    166    149     142   134    122     129    13r    143    143  145  mis 

Hence  the  month  of  Sejjtember  is  about  as  free  from  atmospheric 
disturbances  as  any  portion  of  the  year.  The  period  of  the  autumnal 
equinox  is  quite  as  free  from  storm  and  rain  as  the  days  immediately 
preceding  and  following.  The  wind  movement,  the  rainfall  probability, 
and  the  amount  of  rain  for  the  21st  and  22d  are  all  below  the  average 
for  the  period  from  the  15th  to  the  25th.  In  view  of  the  figures  in  the 
following  table  it  is  difficult  to  find  any  support  for  the  existence  of  a 
particularly  stormy  period  at  the  time  of  the  September  equinox. 


MARYLAND   WEATHER    SERAICE 


481 


WINDS  AND  RAINFALL  FROM  SEPTEMBER  15-25. 
(Average  of  35  jears  at  Baltimore.) 

Wind.  Rainfall. 

Movement.  Probability.  ^^^Aimmnt^^ 

(Miles)  (Per  cent.)  (Inch; 

September  15 139  45  0.60 

16 138  37  0.78 

17 133  40  0.80 

18 130  30  0.15 

19 121  27  0.90 

20 133  28  0.21 

21 130  27  0.15 

22 129  32  0.28 

23 130  35  0.52 

24 133  25  0.30 

25 134  30  0.43 

Average 132  32.4  0.47 

The  months  of  October  and  Xovember  give  us  some  of  the  most  delight- 
ful da3s  of  the  year — days  with  soft,  balmy  atmosphere,  light  southerly 
winds,  cloudless  skies  with  warm  sunshine  during  mid-day,  and  cool 
crisp  nights — the  days  of  the  American  Indian  Summer. 

In  Maryland  light  frosts  make  their  first  appearance  about  the  middle 
of  Octolier,  excejjting  in  the  mountain  districts  where  they  are  earlier, 
while  heavy  frosts  are  usually  delayed  to  the  early  days  of  Xovember. 
The  first  snow  arrives  about  the  middle  of  Xovember. 

In  the  fall  months  the  process  of  redistribution  of  barometric  pressure 
over  ocean  and  continent  is  the  reverse  of  that  of  the  spring.  The  sum- 
mer condition  of  high  barometric  pressure  over  the  Atlantic  Ocean  and 
low  pressure  over  the  central  continental  area  is  gradually  broken  up, 
the  pressure  rising  over  the  continent  and  falling  over  the  ocean.  In 
this  process  of  redistribution  of  pressure  which  is  brouglit  about  by  tlie 
retreat  of  the  sun,  the  sluggisli  atmospheric  movements  of  the  summer 
months  give  way  to  a  more  active  circulation.  Well  defined  areas  of 
high  and  low  pressure  increase  in  frequency.  The  Middle  Atlantic 
states  are  alternately  brouglit  under  tlic  influence  of  the  Atlantic  high 
area  with  its  southerly  winds  and  clear  skies,  and  the  cool  dry  northwest 
winds  of  the  growing  continental  hii:!!  area,  but   it  is  not  until  late  in 


482  THE    CLIMATE    OF   BALTIMORE 

December  that  the  continental  high  area  gains  control  over  the  weather 
situation  in  Maryland,  and  settled  winter  conditions  may  be  expected. 

Indian   Summer. 

In  discussing  weather  types  in  the  preceding  pages  we  have  found  a 
natural  division  into  cyclonic  and  anti-cyclonic  weather — or  the  weather 
are  relatively  low,  and  those  associated  with  relatively  high  barometric 
pressure.  The  weather  conditions  attending  these  moving  or  shifting 
pressure  areas  vary  with  the  season,  or  rather  with  the  annual  increase 
and  decrease  in  temperature  resulting  from  changes  in  the  declination 
conditions  associated  with  large  areas  over  which  the  barometer  readings 
of  the  sun.  But  in  all  seasons  high  areas  are  attended  by  comparatively 
clear  skies  and  moderate  winds,  while  low  areas  bring  clouds  and  rain 
and  relatively  high  winds — they  are  respectively  "  fair  weather  "  and 
"  foul  weather  "  types.  The  temperatures  of  a  given  locality  associated 
with  these  types  depend  upon  the  season  of  the  year  and  the  relative  posi- 
tion of  the  center  of  the  high  or  low  area  with  respect  to  the  locality 
in  question.  The  relative  distribution  of  pressure  determines  the  wind 
direction  while  the  wind  direction  determines  the  temperature.  In  our 
latitudes,  and  especially  over  large  continental  areas  with  the  broad  ocean 
of  equable  temperatures  to  the  east,  a  north  or  northwest  wind  is  a 
relatively  cold  wind,  a  south  or  southeast  wind  is  relatively  warm,  at  all 
seasons  of  the  year,  while  east  winds  and  west  winds  bring  intermediate 
temperatures.  Hence  an  area  of  high  pressure  to  the  west  or  northwest 
of  the  Middle  Atlantic  states,  for  instance,  with  its  resulting  northwest 
to  north  winds  gives  to  this  section  in  all  seasons  a  temperature  below 
the  seasonal  average,  the  amount  of  departure  from  the  normal  depend- 
ing upon  the  intensity  of  development  of  the  high  area.  With  a  high 
area  to  the  east  or  southeast  the  winds  blowing  out  of  the  high  area  and 
over  the  Middle  Atlantic  states  are  from  the  southeast  to  southwest — 
winds  which  are  at  all  seasons  warmer  than  the  seasonal  average.  On 
the  other  hand  a  low  area  to  the  west  or  north  brings  warm  southerly 
winds;  when  it  is  to  the  south  or  east  the  winds  are  from  the  colder 
areas  of  the  north  and  northwest. 


MARYLAND   WEATHER   SERVICE  483 

Under  normal  conditions  there  is  a  constant  succession  of  these  high 
and  low  areas  across  the  country,  approximately  from  the  west  towards 
the  east,  with  an  irregular  cycle  of  two,  three,  or  four  days.  Sometimes 
these  types  are  quite  persistent,  the  pressure  distribution  remammg 
practically  unaltered  for  a  week  or  ten  days,  or  longer  in  exceptional 
cases.  In  mid-summer  an  area  of  high  pressure  developing  over  the 
South  Atlantic  states,  or  over  the  Atlantic  just  off  the  coast,  is  apt  to 
persist  for  many  days  with  only  slight  changes  in  outline  or  intensity, 
resulting  in  "  hot  spells  "  of  greater  or  less  degree,  depending  upon  the 
intensity  of  development  and  persistence  of  this  high  area  to  the 
southeast. 

A  similar  type  of  high  area  frequently,  though  not  annually,  develops 
late  in  the  fall  after  the  summer  has  passed  and  after  the  first  frosts  have 
announced  the  approach  of  winter.  The  development  may  occur  in  the 
latter  part  of  October  or  in  November,  sometimes  even  later  in  the 
season,  and  controls  the  weather  conditions  in  the  Central,  Middle 
Atlantic  and  New  England  states  for  several  days,  or  at  rare  intervals, 
for  several  weeks. 

During  the  periods  in  which  these  southeast  anti-cyclonic  areas  prevail, 
light  dry  southerly  winds  blow  over  the  Middle  Atlantic  states,  the  New 
England  states  and  tlie  Central  states,  there  is  an  absence  of  clouds, 
haze  increases  in  amount,  as  is  usual  during  warm  pei'iods  with  a  sluggish 
movement  of  the  air;  the  mid-day  temperatures  are  high,  while  the 
nights  are  cool.  Separated  from  the  long  season  of  hot  sultry  summer 
weather  by  occasional  incursions  of  the  cold  crisp  air  from  a  northwest 
high  area,  these  periods  of  Indian  Summer,  or  second  summer,  are  among 
the  most  delightful  days  of  the  year.  They  constitute  a  temporary 
halt  in  the  steady  seasonal  fall  of  temperature  and  the  approach  of  real 
winter  weather.  There  are  similar  periods  in  European  weather  but 
there  the  characteristic  charm  of  the  American  Indian  Summer  appears 
to  be  less  pronounced ;  of  such  among  others  are  St.  Martin's  Summer  of 
England  ;  the  Summer  of  St.  Denis  in  Eranee;  and  in  Cicrmany  the  "  Alt- 
weibersommer,"  or  the  "  old  woman's  summer." 


484 


THE    CLIMATE    OF   BALTIMORE 


A  most  interesting  and  instrnetive  account  of  the  occurrence  of  the 
term  "  Indian  Summer  "  in  the  literature  of  the  early  writers  on  Anierica 
is  presented  by  Mr.  Albert  Matthews '  of  Boston.  The  author  finds  after 
an  exhaustive  search  in  books  on  travel  in  North  America  that  "  it  is 
not  until  the  year  1794  that  the  expression  '  Indian  Summer '  occurs 
at  all,  and  not  until  the  nineteenth  century  that  it.  became  well 
established." 


Fig.  169.— The  Weather  of  October  29,  1903   (Indian  Summer), 


The  earliest  use  of  the  term  found  by  Mr.  Matthews  is  in  the  following 

journal  entry  by  Major  Ebenezer  Denny "  while  at  Le  Boeuf,  a  few  miles 

from  the  present  city  of  Erie,  Pa.,  on  October  13,  1794 : 

"  Pleasant  weather.     The  Indian  Summer  here.     Frosty  nights." 

There  is  very  little  agreement  among  writers  who  used  the  term  as  to 

the  time  of  occurrence  of  the  Indian  Summer,  or  as  to  the  length  of  the 

^  Albert  Matthews.     The  term   Indian  Summer.     Monthly  Weather  Review 
for  January  and  February,  1902  (Washington,  D.  C). 
=  Military  Journal.  1859,  p,  198. 


MAUYLAND   WEATHER    SERVICE  485 

period.  This  is.  however,  not  surprising  in  view  of  the  fact  that  the 
type  of  pressure  distribution  which  causes  the  characteristic  weather 
may  develop  at  any  time  of  the  year. 

The  meteorological  conditions  prevailing  over  the  country  on  the 
morning  of  the  29th  of  October,  1903,  at  the  beginning  of  a  brief  period 
of  Indian  Summer  in  the  Middle  Atlantic  states  are  shown  in  the  accom- 
panying weather  chart.  An  area  of  high  barometric  pressure,  centered 
over  the  Upper  Mississippi  Valley  on  the  26th  of  October,  moved  slowly 
southeastward  during  the  26th,  27th,  and  28th,  and  remained  with 
slight  changes  of  configuration  and  position  over  the  Middle  Atlantic 
and  South  Atlantic  states  until  November  4,  when  it  gave  way  to  an  area 
of  low  pres.sure  over  the  Lake  region,  bringing  general  rains  to  tlie 
Atlantic  coast  states. 

On  the  morning  of  October  29  the  center  of  the  high  area  was  over 
Virginia.  The  skies  were  clear  throughout  the  Middle  Atlantic  states, 
the  winds  were  prevailingly  from  the  southwest  and  light.  (See  Fig. 
169.) 

The  following  extracts  from  the  records  of  the  Baltimore  Office  of  the 
United  States  Weather  Bureau  indicate  the  general  character  of  the 
weather  during  the  brief  period  of  Indian  Summer  weather  from  October 
28  to  November  4,  1903: 

October  28.  1903.  Clear  day.  Warmer  and  pleasant;  maximum  61°,.  min- 
imum 40°.  Light  fog  in  morning.  Wind  was  brisk  at  times  during  the  day. 
Wind  west  and  southwest.  Average  velocity  9  miles  per  hour;  maximum 
velocity  22  miles  from  the  west  at  9.10  a.  m. 

October  29,  1903.  Clear  day.  Somewhat  warmer;  maximum  68°,  min 
imum  45°.  Pleasant.  Light  fog  at  8  a.  m.  Wind  from  southwest  to  north- 
west; average  velocity  8  miles. 

October  30,  1903.  Clear  day.  Some  cirrus  clouds  all  day.  Somewhat 
warmer;  maximum  75°,  minimum  52°.  Pleasant.  Light  haze  at  8  a.  m. 
Wind  from  southwest  to  northwest;  average  velocity  5  miles. 

October  31,  1903.  Partly  cloudy  day.  Sky  a  little  more  than  half  clouded 
all  day.  Somewhat  cooler;  maximum  68°,  minimum  44°.  Mild  and  delight- 
ful weather  continues.  Light  fog  in  morning;  light  fog  and  smoke  at  8  p.  m. 
Wind  variable,  from  southeast  to  northwest;   average  velocity  3  miles. 

November  1,  1903.  Clear  day.  Some  cirrus  present  nearly  all  day.  Some- 
what warmer;  maximum  74°,  minimum  46°.     The  mild  pleasant  Indian  Sum- 


486  THE    CLIMATE    OF   BALTIMORE 

mer  weather  continues.  Light  fog  in  morning.  Wind  from  west  to  north- 
west; average  velocity  4  miles. 

November  2,  1903.  Partly  cloudy  day.  Morning  clear;  mid-day  partly 
cloudy,  and  sky  overcast  by  late  afternoon;  evening  clear.  Warm  in  morning, 
cooler  in  afternoon;  maximum  68°.  Mild  and  pleasant.  Light  fog  in  morning 
and  light  smoke  in  evening.    Winds  variable;  average  velority  4  miles. 

Noveinber  3,  1903.  Clear  day.  Warmer  in  afternoon;  maximum  75°,  min- 
imum 48°.  Continued  mild  and  pleasant  weather.  Light  fog  in  morning; 
light  smoke  at  8  p.  m.;  and  light  haze  all  evening.  Wind  west  to  northwest; 
average  velocity  5  miles. 

November  4,  1903.  Partly  cloudy  day.  Morning  clear;  sky  became  nearly 
overcast  with  light  clouds  just  after  noon,  and  so  continued.  Warm  and 
pleasant  weather  continues;  rather  humid  in  afternoon.  Light  rain  began 
11.30  p.  m.  and  continued  at  midnight;  amount  to  midnight,  0.02  inch.  Light 
fog  in  morning,  light  haze  in  evening.  Lunar  corona  seen  at  8  p.  m.,  and  a 
beautiful  lunar  halo  of  22^2°  radius  seen  at  9.15  p.  m.  and  at  10  p.  m.  Wind 
variable;   average  velocity  3  miles. 

The  Variability  of  Autumn  Temperatures. 

The  extreme  ranges  of  the  temperature  conditions  during  the  fall 
months  are  shown  in  detail  in  the  discussion  of  climatic  conditions  in 
Part  I  of  this  report;  these  statistical  tables  are  supplemented  by  the 
records  of  weather  conditions  on  two  selected  typical  fall  days,  namely, 
October  1  and  Thanksgiving  Day,  from  1871  to  1907,  and  on  a  State 
holiday,  September  12,  the  anniversary  of  the  battle  of  North  Point.  The 
variability  of  the  weather  of  the  fall  does  not  differ  materially  from  that 
of  the  spring — both  seasons  are  transitional  periods  connecting  the 
extreme  conditions  of  winter  and  summer.  A  close  study  of  these  tables 
based  on  Baltimore  observations  for  37  3'ears  will  give  a  fair  idea  of  the 
variability  of  fall  weather  over  the  Coastal  Plain  of  the  Middle  Atlantic 
states. 

The  Weather  of  September  12. 
The  12th  day  of  September,  or  Defenders'  Day,  is  a  state  holiday  in 
Maryland  commemorating  the  battle  of  North  Point  in  1814.  In  the 
past  37  years  the  weather  on  this  day  has  been  mostly  clear  to  partly 
cloudy,  with  an  average  temperature  of  about  72°,  and  extremes  rang- 
ing between  93°  and  51°.  The  winds  have  been  light  easterly,  while 
rain  has  occurred  on  an  average  once  in  three  years.  The  distribution 
of  rainfall  has  been  peculiar,  having  occurred  12  tim-es  during  the  first 
20  years  and  but  once  in  the  succeeding  17  years. 


MARYLAND    WEATHER   SEKVICE 


487 


THE    WEATHER    OF    SEPTEMBER    12. 


Year. 
1871 

Max,           Min. 
Temp.       Temp. 
(Degrees  Fahr 
69              64 

Mean 
Temp. 

) 
66 

Character 
of  Day. 

Cloudy 

Wind 
Direction. 

SE 

Daily  Wind 
Movement. 
(Miles) 
70 

Precip 
itation 
(Inch) 
0.01 

1872 

81 

72 

76 

Cloudy 

S 

171 

0.24 

1873 

78 

58 

68 

Clear 

N&SE 

100 

1874 

90 

70 

80 

Ft.  cldy 

SE 

61 

1875 

70 

52 

61 

Cloudy 

E 

143 

1876 

67 

57 

62 

Cloudy 

NE 

140 

0.01 

1877 

74 

69 

72 

Cloudy 

E 

122 

0.89 

1878 

80 

69 

74 

Cloudy 

E 

174 

1879 

72 

51 

62 

Clear 

S 

81 

18S0 

77 

56 

66 

Clear 

SE 

77 

1881 

81 

69 

75 

Clear 

N&,  NW 

107 

0.01 

1882 

72 

61 

66 

Clear 

N 

183 

0.03 

1883 

63 

56 

60 

Cloudy 

NE 

281 

1.13 

1884 

78 

67 

72 

Pt.  cldy 

NB 

117 

. . . 

1885 

72 

62 

67 

Pt.  cldy 

NE-SE-S 

95 

1886 

86 

61 

74 

Pt.  cldy 

NE-SW-W 

129 

0.53 

1887 

77 

63 

70 

Cloudy 

SW 

118 

0.64 

1888 

84 

62 

73 

Clear 

SW 

158 

0.01 

1889 

69 

64 

66 

Cloudy 

NE 

368 

0.55 

1890 

82 

71 

76 

Cloudy 

S 

171 

1.06 

1891 

76 

60 

68 

Clear 

NE&S 

128 

1892 

79 

61 

70 

Pt.  cldy 

SB 

200 

1893 

70 

60 

65 

Cloudy 

B 

239 

1894 

73 

55 

64 

Clear 

E 

130 

1895 

93 

73 

83 

Clear 

W 

126 

1896 

80 

67 

74 

Cloudy 

B&SW 

62 

1897 

79 

67 

73 

Cloudy 

E 

173 

1898 

74 

53 

64 

Clear 

N&NW 

155 

1899 

82 

57 

70 

Clear 

SW&NW 

105 

1900 

88 

70 

79 

Clear 

SW&NW 

165 

1901 

87 

69 

78 

Pt.  cldy 

SW 

119 

0.01 

1902 

75 

57 

66 

Clear 

s 

153 

1903 

84 

G9 

76 

Clear 

E 

154 

1904 

87 

62 

74 

Clear 

N 

123 

1905 

75 

63 

69 

Cloudy 

NW 

138 

1906 

88 

72 

80 

Pt.  cldy 

S&SW 

151 

... 

Average 

78 

63 

72 

144 

0.39 

32 

c 

00        f^ 

9     = 
o      if 

> 
> 

5  C 

>     6      o 

O          irt         O          iTi 

«^j      -      —      d     o 

■^.      ■■ 

1  ■  , 

1 

'' C\''    '    ' 

;  %y^ 

; 

> 

F-^ 

/    \ 

\ 

iVT": 

i^  i 

i 

o 
o 

f 

(  ( 

1 

\ 

\^ 

\    i 

^ 

! 

~az- 
CD 

T. 

UJ 

IZ) 

UJ 

o. 

Z 

L.J 

V 

^  X 

/ 

■<: 

y'^ 

( 

,^ 

■f^  i.  ^ 

CO 

CD 

K 

: 

; 

« 

s 

^-s 

^s 

y 

* 

UJ 

:y:  T  ; 

\  , 

? 

\ 

:Y''   *  • 

■rn'  i  ■ 

^ 

// 

\ 

a: 

:%\i\ 

z 

1 

;2:J: 

o 

^  i 

f  /{/ 

/ 

:1 

'Vij'; 

s^ 

\^ 

'^'  "^ 

i 

V 

wP 

\    : 

: 

;^:      .H  : 

! 

\ 

S^ 

)    ; 

M 

•^'  y  : 

/ 

^/ 

\ 

V 

:T'  >: 

^ 

N 

N,\ 

I   ; 

/ 

V 

■  ^  i)^ 

2 

/?/ 

/ 

X  ; 

1 

Hv 

?^V 

) 

*  : 

;   ^ 

^ 

Q- 

n  . 

^<^'/ 

/ 

^'■ 

o 

i 

/  : 

; 

£ 

;  ■ 

^"^' 

y  ^ 

^i 

s 

o 

/  /  f' 

J^ 

> 

;C)":*1      : 

-i- 

UJ 

O- 

"uJ 

:/; 

■i'  1  i 

V 

\^: 

r 

:    1 

\    \ 

2Y'   '    - 

2 

X^ 

i: 

\ 

\ 

"A: 

<^ 

^^^ 

/'; 

: 

: 

p" 

^N 

: 

/ 

1    :    1     ; 

■  "^    '• 

00 

■  -   ; 

■ 

-'—J 

Ti'i  ■ 

:  ^     ■■ 

CI        — ' 


-      <d 


ilARYLAXD   WEATHER    SERVICE  489 

The  Weather  of  October  1. 

The  first  day  of  October  falls  within  a  period  likely  to  have  some  of 
the  most  delightful  weather  of  the  year.  With  an  early  morning  tem- 
perature of  about  54°  and  an  afternoon  temperature  of  72°,  the  average 
for  the  day  is  63°.  The  highest  temperature  for  the  day,  occurring  in 
1889,  was  89°,  and  the  lowest  was  39°,  recorded  in  1899.  The  record  of 
cloudiness  shows  a  high  percentage  of  bright  clear  days :  There  were 
nineteen  clear  days,  eleven  partly  cloudy  days,  and  but  five  cloudy  days 
in  the  entire  period  from  1871  to  1907.  Eain  occurred  on  but  seven 
days  of  the  37  anniversaries,  while  the  precipitation  ^as  evenly  distrib- 
uted through  the  period.  The  early  part  of  October  is  the  period  of  the 
year  most  free  from  rain.  The  winds  have  been  light  and  mostly  from 
a  northerly  direction. 

The  Weather  of  Thanksgiving  Day. 

Cloudy  to  partly  cloudy  weather,  with  moderate  winds  from  the  north- 
west, has  prevailed  on  this  day  during  the  past  37  years  in  Baltimore. 
The  mean  temperature  for  the  day  has  been  about  40°,  with  an  early 
morning  temperature  near  the  freezing  point,  rising  to  about  46°  in 
the  afternoon.  The  extremes  for  the  day  have  ranged  between  71°  in 
1896,  to  21°,  in  1903.  Precipitation  was  evenly  distributed  through 
the  period,  and  occurred  on  an  average  once  in  three  years,  generally  in 
the  form  of  rain.  The  amounts  have  been  light,  mostly  less  than  a 
(piarter  of  an  inch. 


490 


THE    CLIMATE    OF   BALTIMORE 


THE  WEATHER  OF  OCTOBER  1. 


Year. 
1871 

Max.          Min. 
Temp.       Temp. 
(Degrees  Fah 
69              50 

Mean 

Temp. 
r.) 
60 

Character 
of  Day. 

Clear 

Wind 
Direction. 

NW 

Dailj'  AVind 
Movement. 
(Miles) 
80 

Precip- 
itation. 
(Inch) 

1872 

66 

53 

60 

Pt.  cldy 

NW 

125 

1873 

66 

47 

56 

Pt.  cldy 

SW 

153 

1874 

64 

48 

56 

Clear 

SW 

233 

1875 

72 

52 

62 

Pt.  cldy 

NW 

133 

0.08 

1876 

60 

45 

52 

Pt.  cldy 

w 

100 

0.40 

1877 

75 

54 

64 

Pt.  cldy 

N 

85 

1878 

73 

54 

64 

Clear 

NE&SE 

89 

1879 

80 

56 

68 

Clear 

SE 

61 

1880 

64 

43 

54 

Clear 

SE 

112 

1881 

89 

72 

80 

Clear 

W 

132 

1882 

75 

60 

68 

Clear 

N 

61 

1883 

64 

56 

60 

Pt.  cldy 

N&SE 

148 

1884 

84 

69 

76 

Pt.  cldy 

SE 

67 

1885 

76 

59 

68 

Cloudy 

SE 

104 

0.31 

1886 

66 

48 

57 

Clear 

NW 

187 

1887 

76 

65 

71 

Pt.  cldy 

W 

74 

1888 

71 

46 

59 

Pt.  cldy 

SW 

265 

0.04 

1889 

78 

61 

70 

Cloudy 

SW 

162 

1890 

70 

51 

60 

Pt.  cldy 

NB 

81 

0.81 

1891 

69 

51 

60 

Cloudy 

NE&SE 

200 

1892 

83 

58 

70 

Clear 

NE&S 

213 

1893 

66 

45 

56 

Clear 

NW 

184 

1894 

74 

58 

66 

Clear 

NW 

137 

1895 

63 

42 

52 

Clear 

NW 

178 

1896 

70 

58 

64 

Pt.  cldy 

W 

96 

1897 

88 

58 

73 

Clear 

NW 

57 

1898 

78 

58 

68 

Clear 

NE&E 

164 

1899 

56 

39 

48 

Clear 

NW 

117 

1900 

70 

65 

68 

Cloudy 

NE 

174 

0.08 

1901 

71 

55 

63 

Clear 

SE&NW 

80 

1902 

77 

64 

70 

Pt.  cldy 

NW 

192 

1.11 

1903 

79 

53 

66 

Cloudy 

SW 

133 

1904 

73 

54 

64 

Clear 

W 

287 

1905 

87 

59 

73 

Clear 

w 

68 

1906 

57 

52 

54 

Cloudy 

NE 

246 

Average 

72 

54 

63 

138 

0.40 

:\rARYLAXD   WEATHER    SERVICE 


491 


•THE   WEATHER  OF  THANKSGIVING   DAY. 


Year. 

Date 
Nov. 

Max. 
Temp. 

MiQ. 
Temp 

Mean 
Temp. 

Character 
of  Day. 

Wind     Dailj 
Direction,  ifov 

•  Wind 
ement. 

Precip- 
itation. 

(Be 

grees  Fahr.) 

Miles) 

(Inch) 

1871 

30 

35 

29 

32 

Cloudy 

NW 

255 

1872 

28 

41 

29 

35 

Pt.  cldy 

SB 

106 

1873 

27 

52 

34 

43 

Pt.  cldy 

SW 

141 

1874 

26 

41 

28 

34 

Clear 

N 

1875 

25 

41 

30 

36 

Clear 

SE 

134 

1876 

30 

39 

25 

32 

Cloudy 

N 

97 

1877 

29 

51 

33 

42 

Pt.  cldy 

NW 

149 

0.01 

1878 

28 

52 

44 

48 

Cloudy 

NW 

249 

0.01 

1879 

27 

51 

33 

42 

Pt.  cldy 

SE 

71 

1880 

25 

36 

30 

33 

Cloudy 

N 

83 

0.18 

1881 

24 

41 

28 

34 

Pt.  cldy 

NW 

272 

0.14 

1882 

30 

36 

28 

32 

Clear 

NW 

208 

1883 

29 

43 

34 

38 

Clear 

S 

106 

1884 

27 

54 

31 

42 

Cloudy 

SW 

111 

1885 

26 

44 

35 

40 

Cloudy 

NW 

223 

1886 

25 

50 

34 

42 

Cloudy 

NW 

224 

1.07 

1887 

24 

48 

45 

46 

Cloudy 

NE-E-NW 

77 

0.01 

1888 

29 

46 

37 

42 

Cloudy 

W 

177 

1889 

28 

56 

43 

50 

Pt.  cldy 

SW&NW 

153 

0.69 

1890 

27 

37 

32 

34 

Cloudy 

N&NW 

133 

1891 

26 

44 

38 

41 

Cloudy 

NE&E 

126 

0.08 

1892 

24 

36 

21 

28 

Clear 

W 

392 

1893 

30 

54 

46 

50 

Clear 

sw&w 

148 

1894 

29 

35 

24 

30 

Clear 

N 

130 

1895 

28 

48 

31 

40 

Clear 

N 

80 

1896 

20 

71 

49 

60 

Pt.  cldy 

SE 

73 

1897 

25 

46 

32 

39 

Pt.  cldy 

SE 

70 

1898 

24 

35 

30 

32 

Cloudy 

N 

177 

0.20 

1899 

30 

62 

41 

52 

Pt.  cldy 

SE 

60 

1900 

29 

52 

40 

46 

Cloudy 

SE 

70 

1901 

28 

33 

25 

29 

Clear 

W 

135 

1902 

27 

49 

39 

44 

Pt.  cldy 

NW 

236 

0.11 

1903 

26 

33 

21 

27 

Pt.  cldy 

NE 

114 

1904 

24 

54 

37 

46 

Pt.  cldy 

NW 

226 

1905 

30 

49 

25 

37 

Pt.  cldy 

NW 

336 

1906 

29 

45 

33 

39 

Clear 

NW 

223 

1907 

28 

57 

43 

50 

Cloudy 

SW 

173 

Average 


46 


33 


40 


159 


0.25 


492  THE    CLIMATE    OF   BALTIMORE 

THE  HEAVY  RAINS  OF  SEPTEMBER  34-26,  1902. 

The  closing  week  of  September  was  marked  by  heavy  and  long  con- 
tinued rains  throughout  the  Middle  Atlantic  and  New  England  states. 
The  period  preceding  had  been  one  of  deficient  precipitation  in  Mary- 
land, but  the  rains  of  the  24th,  25th,  and  26th,  and  the  heavy  shower 
of  the  30th,  gave  the  month  a  total  fall  far  in  excess  of  the  normal  Sep- 
tember amount. 

From  the  16th  to  the  22 d  the  winds  were  mostly  from  the  northeast 
and  east.  On  the  afternoon  of  the  23d  there  was  a  change  to  southeast 
and  southwest,  which  direction  prevailed  to  noon  of  the  24th,  when  it 
again  returned  to  northeast.  Eain  set  in  about  nine  o'clock  at  night  of 
the  24th  and  increased  rapidly  in  intensity.  By  midnight  about  one 
inch  had  fallen.  During  the  remainder  of  the  night  only  a  trace  fell, 
the  wind  continuing  from  the  northeast.  From  8  a.  m.  of  the  24th  until 
5  p.  m.  of  the  26th  the  rainfall  continued  with  scarcely  any  interruption. 
A  change  in  the  wind  from  northeast  to  southeast  at  8  p.  m.  of  the  24th 
was  accompanied  by  a  sudden  increase  in  intensity,  and  heavy  showers 
continued  to  fall  throughout  the  night.  Late  in  the  afternoon  of  the 
26th  the  rain  ceased,  with  a  change  in  the  direction  of  the  wind  to 
northeast. 

The  total  amount  of  rainfall  for  the  three  days  as  measured  at  the 
station  of  the  United  States  Weather  Bureau  was  5.29  inches.  About 
one  inch  of  this  amount  fell  during  the  showers  of  the  24th,  lasting  about 
three  hours;  the  remainder  fell  during  a  continuous  rain  of  nearly  36 
hours.  The  rate  of  fall  was  never  excessively  heavy,  but  the  storm  was 
remarkable  for  the  long  uninterrupted  fall  of  rain. 

During  the  entire  period  the  winds  were  light,  exceeding  10  miles  an 
hour  for  only  a  few  hours,  with  a  maximum  velocity  of  16  miles,  and 
an  average  velocity  of  about  6  miles.  The  barometer  was  remarkably 
steady.  There  was  no  decided  fall,  but  only  a  slow  rise  from  midnight 
of  the  23d  to  midnight  of  the  24th,  and  then  a  slow  but  steady  fall  to 
the  27th,     The  heavy  shower  of  the  24th  fell  with  a  rising  barometer. 

The  daily  weather  charts  of  the  24th  to  the  27th  show  an  unusually 
sluggish  movement  of  the  areas  of  high  and  low  pressure.     An  anti- 


MARYLAND    WEATHER    SERVICE  493 

cyclone  prevailed  over  the  Lake  region  on  the  2-ith,  and  then  moved  slowly 
eastward  and  remained  nearly  stationary  over  the  New  England  states 
for  two  or  three  days.  In  the  Mississippi  Valley  the  pressure  was  com- 
paratively low,  with  a  long  trough-shaped  depression  which  did  not 
develop  into  a  well  defined  storm  center  until  after  the  occurrence  of  the 
heavy  rains  at  Baltimore. 


FOEETELLING  THE  WEATHER.     (Historical.) 

INTRODUCTION. 

If  we  are  to  believe  the  statements  of  certain  Greek  philosophers  and 
historians,  the  art  of  foretelling  the  weather  is  a  lost  art.  In  Aristotle's 
Politics  we  find  this  anecdote  related  about  Thales  of  Miletus,  the  wisest 
of  the  seven  sages  of  Greece :  Thales  was  apparently  accused  by  his 
neighbors  of  lacking  the  ability  or  the  energy  to  make  money.  Desiring  to 
convince  them  that  his  poverty  was  due  to  choice  and  not  to  necessity, 
and  perceiving  by  his  skill  in  astrology,  during  the  winter,  that  there 
would  be  a  great  crop  of  olives  that  year,  he  contrived  to  hire  at  small 
cost  all  the  oil  presses  in  Miletus  and  Chios,  as  there  was  no  one  to  bid 
against  him.  When  the  season  came  for  making  oil  and  the  demand 
for  presses  was  great,  he  disposed  of  them  at  his  own  figures,  as  the  entire 
supply  was  in  his  possession.  Thus  he  convinced  his  slanderers  that  it 
was  easy  for  philosophers  to  be  rich  if  they  desired  to  be,  but  that  this 
was  not  their  aim  in  life. 

During  the  twenty-five  liundred  years  since  Tliales  is  credited  with 
having  cornered  olive  presses  as  a  result  of  a  seasonal  forecast  of  the 
weather,  there  have  been  many  successful  corners  of  the  markets,  but 
the  operators  have  passed  from  Academic  groves  to  the  Stock  Exchange. 
The  long  range  prophet  of  to-day  has  lost  caste  with  his  contemporaries 
and  apparently  lacks  the  ability  to  enrich  himself  by  plying  his  vocation. 
After  centuries  of  effort  to  foresee  changes  in  the  weather  about  us  we 
must  still  be  content  with  an  imperfect  glimpse  one,  two.  or  n\  most. 
three  to  four  days  into  the  future. 


494  THE    CLIMATE    OF   BALTI]\IORE 

The  methods  employed  in  earliest  historic  times  in  foretelling  the 
weather  are  in  vogue  to-day,  though  the  accumulated  experience  of  cen- 
turies of  close  observation  of  weather  sequences  has  made  some  of  them 
more  accurate,  while  the  result  of  modern  research  and  the  use  of  the 
electric  telegraph  have  added  a  new  method  which  is  now  firmly  estab- 
lished in  the  ofl&cial  organizations  of  all  civilized  nations.  There  was 
probably  a  time  in  the  development  of  all  races  when  the  weather  was 
believed  to  be  under  the  arbitrary  rule  of  weather  deities  or  of  evil  spirits. 
With  the  progress  of  civilization  and  a  closer  observation  of  the  elements 
the  belief  in  fixed  laws  or  sequences  in  weather  conditions  became  firmly 
established  in  the  minds  of  the  philosophers  of  the  time,  and  the  search 
for  causes  was  begun. 

NATURAL  SIGNS. 

As  early  as  the  time  of  Homer  (about  950  B.  C.)  the  four  principal 
winds  were  named  and  their  characteristics  noted  for  the  vicinity  of 
Greece :  Boreas  was  the  cold  stormy  north  wind ;  Euros  the  clear 
and  bright  easterly  wind;  Notos  the  moist  south  wind;  and 
Zephyros  the  glorious  west  wind  which  brings  on  the  spring. 
A  manuscript  of  the  time  of  Kero  (37-68  A.  D.)  is  said  to  contain  a  rec- 
ord of  the  probable  weather  for  nearly  every  day  in  the  year,  especially 
as  to  wind  direction,  which  would  indicate  that  at  that  time  regular 
observations  of  the  weather  must  have  been  made  in  Eome.^  Prognostics 
based  upon  a  close  observation  of  the  weather  and  shrewd  inference  as  to 
sequences,  form  a  valuable  part  of  the  literature  of  weather  probabilities; 
they  are  built  upon  a  firm  basis,  but  require  discrimination  in  their  appli- 
cation. If  traced  to  the  region  of  their  origin  many  would  doubtless 
be  found  to  apply  with  considerable  accuracy  to  actual  conditions  in  the 
locality  for  which  they  were  formulated.  Eemoved  from  their  original 
horizon  and  applied  to  remote  regions  they  may  fail  to  have  any  sig- 
nificance. It  is  not  to  be  expected  that  the  proverbs  of  the  Greeks  and 
Eomans  as  collected  by  Aristotle,  Theophratus,  Aratus,  and  Pliny,  and 

'  Hellman,  G.  Die  Anfange  der  meteorologischen  Beobachtungen,  8.  Berlin, 
1893. 


MARYLAND   WEATHER   SERVICE  495 

which  may  aptly  fit  the  climatic  conditions  of  Southern  Europe  should 
prove  of  the  same  value  when  transplanted  to  the  totally  different  cli- 
mates of  ^^o^thern  Europe,  Great  Britain  and  North  America.  Yet  a 
large  number  of  the  proverbs  current  to-day  in  this  country  can  be  traced 
back  through  the  English  and  German  folk  literature  to  early  Greek  and 
Eoman  weather  proverbs.  These  proverbs  have  been  modified  and  added 
to  from  time  to  time  as  observation  or  fancy  have  dictated.  Many  of 
them  have  a  common  application  in  widely  scattered  portions  of  the 
globe  and  find  their  interpretation  in  a  study  of  the  modern  synoptic 
weather  charts. 

"  The  moon  in  a  circle  indicates  a  storm." 

"  When  bees  to  a  distance  wing  their  flight, 
Days  are  warm  and  skies  are  bright; 
But  when  their  flight  ends  near  their  home. 
Stormy  weather   is  sure  to  come." 

"  Do  business  with  men  when  the  wind  is  from  the  northwest." 

In  the  temperate  regions  of  the  earth  there  is  an  eastward  drift  of 
storms  and  general  weather  conditions.  The  advance  of  a  storm  area 
is  marked  by  increasing  humidity,  by  characteristic  cloud  formations, 
systematic  changes  in  the  direction  of  the  wind,  by  certain  optical 
phenomena  like  lunar  and  solar  halos,  and  brilliant  cloud  tints.  The 
increased  humidity  produces  a  greater  sensitiveness  to  changes  in  tempera- 
ture, causes  irritability  and  oppressiveness.  These  facts  were  recognized 
as  signs  of  an  approaching  storm  long  before  the  general  nature  and 
movements  of  storms  were  known. 

"  If  the  rain  falls  during  an  east  wind  it  will  continue  a  full  day." 
"  If  it  rains  before  seven  it  will  stop  before  eleven." 

The  average  rain  lasts  less  than  four  hours,  while  a  rain  storm  begin- 
ning with  an  east  or  northeast  wind  is  likely  to  continue  all  day. 

"  A  year  of  snow,  a  year  of  plenty." 
"  Much  snow,  much  hay." 
"  A  snow  year,  a  rich  year." 

"  Snow  is  the  poor  man's  fertilizer,  and  good  crops  will  follow  a  winter  of 
heavy  snowfall." 


496  THE    CLIMATE   OF   BALTTMOKE 

The  protective  value  of  a  good  covering  of  snow  has  long  been  recog- 
nized by  the  farming  community.  It  supplies  moisture,  and  prevents 
alternate  freezing  and  thawing  of  the  ground,  so  injurious  to  winter 
wheat. 

"  A  late  spring  is  a  great  blessing." 

"  When  gnats  dance  in  February,  the  husbandman  becomes  a  beggar." 

"  It  is  better  to  see  a  troop  of  wolves  than  a  fine  February." 

These  and  similar  proverbs  are  expressions  of  the  harmful  consequences 
of  an  early  spring. 

ASTRO-METEOROLOGY, 

There  is  another  class  of  weather  forecasts  much  less  worthy  of  respect, 
but  supported  by  a  deeply  rooted  and  widely  prevailing  faith  in  the  influ- 
ence of  the  planets  upon  the  weather.  This  popular  belief  in  the  plane- 
tary influence,  especially  as  it  relates  to  the  moon,  though  combated  for 
centuries  will  not  be  downed,  and  it  is  probably  as  firmly  fixed  in  the 
minds  of  men  to-day  as  it  was  in  the  middle  ages,  and  earlier,  and  still 
controls  the  acts  of  many  a  man  from  the  planting  of  corn  to  the  cutting 
of  his  hair, 

"  The  peach  crop  never  fails  when  the  first  blossoms  appear  in  the  light  of 
an  increasing  moon." 

"  If  the  new  moon  appears  with  the  points  of  the  crescent  turned  up  the 
month  will  be  dry;  if  the  points  are  turned  down  it  will  be  wet." 

Belief  in  the  influence  of  the  moon  is  not  unreasonable.  The  sun 
is  too  methodical  in  his  movements  to  account  for  the  abrupt  and  varied 
changes  of  weather  in  our  latitude.  The  great  variety  of  phase  and 
varying  combinations  of  position  shown  by  the  moon  and  planets  sug- 
gested a  ready  explanation  for  any  possible  change  of  weather,  and  we 
find  at  a  very  early  period  a  particular  influence  ascribed  to  each  planet 
and  to  each  phase  of  the  moon. 

"  The  moon  and  the  weather 
May  change  together, 
But  change  of  the  moon 
Does  not  change  the  weather. 


MARYLAND    WEATHER    SERVICE.  497 

If  we'd  no  moon  at  all, 
And  that  may  seem  strange, 
We  still  should  have  weather 
That's  subject  to  change." 

— Notes  and  Queries,  1875. 

According  to  a  well  known  authority  '  on  the  early  literature  of  mete- 
orology, the  Greeks  at  first  employed  only  the  natural  weather  signs, 
such  as  winds,  clouds,  optical  phenomena,  etc.  Later  there  was  added 
a  kind  of  calendar  founded  upon  actual  observations  and  giving  average 
values.  These  were  posted  in  public  places,  and  had  originally  more 
of  climatological  matter  than  of  prophecy.  To  fix  the  dates  astronomical 
phenomena  were  employed,  such  as  the  rising  and  setting  of  stars.  This 
association  of  weather  elements  with  the  stars  and  planets,  intended 
simply  to  indicate  time,  came  to  have  a  causal  connection  attributed  to 
it.  The  error  was  combated  by  some  of  the  Greek  philosophers,  espe- 
cially by  Gemenius'  (about  70  B.  C),  to  whom  is  attributed  the  follow- 
ing passage : 

"  Concerning  the  teachings  of  weather  phenomena,  there  is  a  prevalent  and 
erroneous  superstition  that  atmospheric  phenomena  depend  upon  the  rising 
and  setting  of  the  stars.  Mathematics  and  natural  history  teach  a  totally 
different  conception." 

These  warnings  were  unheeded,  however,  and  under  the  influence  of 
the  Alexandrian  school  astro-meteorology  constantly  won  in  favor.  The 
most  renowned  astronomer  of  ancient  times,  Ptolemy  (about  140  A.  D.), 
was  also  the  greatest  astrologer  and  laid  the  foundation  of  weather 
prophecy  on  astrological  principles.  His  teachings  were  supreme  for 
many  centuries. 

The  Arabians  were  also  ardent  followers  of  astrology  and  they  spread 
their  views  through  Moorish  Spain  into  Christian  lands.  Here  astrology 
obtained  a  firm  footing  among  the  common  people,  under  the  protection 
of  princes  and  the  church.  Toward  the  end  of  the  XII  century  there 
arose  a  custom  of  foretelling  not  only  weather  phenomena,  but  also 
political  and  religious  events  for  a  year  in  advance. 

'  Hellman,  G.     Wetterprognose  und  Wetterberichte,  8.     Berlin,  1893. 


498  TPIE    CLIMATE    OF   BALTIMORE 

SYMBOLIC   DAYS. 

There  is  another  class  of  weather  proverbs  based  upon  time-honored 
superstitions  but  not  so  persistently  belived  in  as  those  based  upon  the 
moon's  influence.  The  belief  that  the  weather  of  the  first  twelve  days 
of  the  new  year  in  some  way  symbolized  the  general  character  of  the 
entire  year  is  very  old.  Traces  of  it  are  found  in  the  customs  of  many 
nations.  In  Christian  lands  these  symbolic  days  have  been  transferred 
to  the  twelve  days  beginning  with  Christmas  and  ending  with  Epiphany, 
while  many  more  such  days  have  been  added  to  the  calendar.  As  examples 
of  this  class  may  be  mentioned  Candlemas  (February  2),  the  familiar 
ground-hog  day,  and  July  15,  St.  Swithin's  day. 

"  If  on  Candelmas  day  it  is  bright  and  clear  the  ground-hog  will  stay  in  his 
den,  indicating  that  more  cold  and  rain  are  to  come;  but  if  it  snows  or  rains, 
he  will  creep  out  as  the  winter  is  ended." 

"  If  Candlemas  be  fair  and  clear 
Ther'll  be  two  winters  in  the  year." 

"  In  this  month  is  St.  Swithin's  Day, 
On  which  if  it  should  rain  they  say. 
Full  forty  days  after  it  will 
Or  more  or  less  some  rain  distil." 

"Three  days  of  September  (20,  21,  22)  rule  the  weather  for  October,  No- 
vember, and  December." 

Of  the  same  fanciful  character  are  the  sayings  which  belong  to  the 
class  with  the  following: 

"  When  squirrels  lay  in  a  large  supply  of  nuts,  expect  a  cold  winter." 
"  A  double  husk  of  corn  indicates  a  severe  winter." 

EARLY  BOOKS   ON"   WEATHER   PROVERBS. 

There  are  two  very  old  booklets  relating  to  weather  proverbs  which 
exerted  a  wide  influence  in  the  16th  and  17th  centuries,  especially  in 
Germany,  though  literal  translations  soon  found  their  way  into  all 
European  nations.  One  of  these  bearing  the  title  "  Reynman's  Weather 
Book  "  first  appeared  in  print  in  1510  and  was  the  earliest  meteorological 
work  printed  in  the  German  language.     It  is  a  little  book  of  about  12 


MARYLAND   WEATHER    SERVICE  499 

pages  dealing  mostly  with  natural  weather  signs  as  is  shown  by  the 
following  table  of  contents: 

Contents  of  Reynman's  "  Wetter  BUchlein." 

(1)  Circles  about  the  sun  and  moon. 

(2)  Colors  qi  the  stars  and  of  shooting  stars. 

(3)  Weather  signs  at  sunrise  and  sunset. 

(4)  The  clouds. 

(5)  The  rainbow. 

(6)  Signs  of  the  seasons. 

(7)  The  new  and  full  moon. 

(8)  The  winds. 

(9)  Hail. 

About  the  same  time  (1508)  there  appeared  a  somewhat  similar 
weather  book  of  the  true  folk-literature  type,  entitled  "  Bauren  Praktik," 
of  which  Dr.  Hellmann  has  noted  about  60  editions  belonging  to  the 
16th J  ITth,  18th,  and  19th  centuries,  in  Germany  alone,  in  addition  to 
a  host  of  translations  which  appeared  in  other  countries.  The  greater 
portion  of  this  little  book  is  devoted  to  forecasts  of  the  weather  for  the 
entire  year  as  imlicated  by  the  weather  of  Christmas  Day  and  succeeding 
days  to  the  5th  of  January. 

Translations  of  these  books  gained  a  wide  circulation  in  England  dur- 
ing the  16th  and  ITth  centuries  under  the  titles  "  The  husbandman's 
practice,  or  prognostication  forever,"  and  "  The  Book  of  Knowledge." 

An  extract  from  the  preface  of  one  of  the  early  English  editions  of  the 
"  Bauren  Praktik  "  is  here  given : 

"  The  wise  and  cunning  masters  in  Astronomy  have  found,  that  man  may 
see  and  mark  the  weather  of  the  holy  Christmas  night,  how  the  whole  year 
after  shall  be  on  his  working  and  doing,  and  they  shall  speak  on  this  wise. 

"  When  on  Christmas  night  and  evening  it  is  very  fair  and  clear  weather, 
and  is  without  wind  and  without  rain,  then  it  is  a  token  that  this  year  will 
be  plenty  of  wine  and  fruit:  But  if  the  contrariwise,  foul  weather  and 
windy,  so  shall  it  be  very  scant  of  wine  and  fruit " 

FORECASTS   BASED  ON    AVERAGE  AND   EXTREME   VALUES. 

The  method  of  average  and  extreme  values  of  weather  conditions  of 
a  given  place  as  a  basis  of  weather  forecasts  has  an  extremely  limited 
application  in  our  latitudes.     There  are  regions  where  the  weather  con- 


500  THE    CLIMATE    OF   BALTIMORE 

ditions  of  an}'  given  day  of  the  year  do  not  vary  greatly  from  the  average 
conditions  for  the  month,  where  one  day  is  very  much  like  another  for 
months  at  a  time,  and  where  a  long-range  forecast  can  safely  be  made. 
Such  conditions  are,  however,  not  found  north  of  latitude  20°  or  25°  in 
the  belt  containing  the  great  bulk  of  the  civilized  nations  of  the  earth. 
Here  changes  in  the  weather  are  so  rapid  and  so  irregular  that  the  figure 
representing  the  average  temperature  for  the  mouth  affords  very  little 
clue  as  to  the  temperature  of  any  given  day  in  the  month.  The  average 
temperature  for  the  month  of  February,  for  example,  at  Baltimore  is 
35°  ;  on  the  23d  of  February,  1874,  the  temperature  rose  to  a  maximum 
of  78°,  on  the  same  day  of  the  month  in  the  year  1873  a  minimum  of 
12°  was  recorded,  and  on  the  10th  of  February,  1899,  during  the  great 
blizzard  it  fell  to  7°  below  zero,  an  absolute  range  of  85°. 

Temperature  Variability. 
The  diagrams  presented  on  Plate  III  and  Plate  IV  show  very  clearly 
the  great  changes  to  which  we  are  subjected  in  Baltimore  during  the 
course  of  the  year.  On  Plate  III  is  shown  the  average  value  of  the 
highest  and  lowest  temperature  for  each  day  for  30  years,  and  the  mean 
between  the  average  maximum  and  average  minimum,  that  is,  the  daily 
normal  temperature  based  on  30  years'  observations.  These  diagrams 
indicate  the  most  probable  readings  of  the  thermometer  to  be  expected 
upon  any  given  day  of  the  year;  but  such  values  may  prove  to  be  a 
delusion  as  a  basis  for  prediction.  On  plate  IV  we  have  the  absolute 
extremes  of  temperature  occurring  upon  each  day  of  the  year  during 
the  past  30  years ;  also  a  line  representing  the  difference  between  the 
extremes,  or,  as  it  is  called,  the  "  extreme  range  "  for  each  day  in  degrees 
Fahrenheit.  This  curve  presents  some  interesting  characteristics  of 
Baltimore  weather.  The  most  noticeable  feature  is  the  great  variability 
during  the  winter  months  as  compared  with  the  summer  months.  The 
greatest  fluctuations  occur  in  February,  although  those  of  March  and 
April  are  not  far  behind. 

Rainfall  Prohahility. 
There  is  no  meteorological  element  so  irregular  in  its  method  of  occur- 
rence, or  in  quantity,  as  rainfall.     Hence  it  is  extremely  hazardous  to 


MARYLAXD   WEATHER   SERVICE  501 

discuss  rainfall  probability.  The  history  of  the  30  years  from  18T1  to 
1902  presents  many  instructive  features,  and  we  may  even  venture  to 
draw  some  inferences  of  a  general  character  as  to  the  future.  On  Plate 
IX  will  be  found  several  diagrams  relating  to  the  probable  occurrence  of 
rain  or  snow  at  Baltimore  for  each  day  of  the  year,  expressed  in  percen- 
tage of  the  possible  number.  For  example,  on  the  5th  day  of  July  it 
rained  15  times  in  the  past  30  years,  or  50  per  cent  of  the  possible  num- 
ber of  days.  This  may  be  considered  as  a  percentage  of  rainfall  proba- 
bility, although  the  figure  has  little  value.  The  value  as  a  prophecy  is 
increased,  however,  if  we  reduce  the  figures  to  5  or  10  day  means. 

SPECIAL   DAYS. 

The  variability  of  weather  conditions  upon  any  given  day  may  also 
be  shown  in  another  way.  In  the  chapter  on  winter  the  weather  of 
each  22d  day  of  February  for  a  long  series  of  years  at  Baltimore  is 
represented.  The  highest  and  lowest  temperatures  of  the  day,  the  height 
of  the  barometer,  the  amount  of  cloudiness,  the  prevailing  wind  direc- 
tion, and  the  rainfall  or  snowfall,  are  shown. 

The  average  conditions  on  the  22d  of  February  would  be  represented  by 
a  maximum  temperature  of  46°,  an  average  temperature  of  36°,  and  a 
minimum  of  32°,  with  a  westerly  wind,  partly  cloudy  weather,  and  with 
little  or  no  precipitation.  During  the  past  37  years  there  were  16  clear 
days,  12  partly  cloudy  days,  and  9  cloudy  days.  Eain  or  snow  fell  dur- 
ing some  portion  of  the  day  or  night  on  13  days,  but  only  on  5  days  after 
9  a.  m. 

Based  upon  such  facts  the  following  calculation  was  made  as  a  matter 
of  curiosity  early  in  February  of  1902  to  "ascertain  the  chances  in  favor 
of  a  fair  day  on  the  25th  anniversary  of  the  opening  of  the  Johns  Hop- 
kins University: 

There  were  2  chances  in  3  in  favor  of  a  clear  or  partly  cloudy  day. 
There  were  3  chances  in  4  in  favor  of  a  day  without  rain  or  snow. 
There  were  8  chances  in  9  in  favor  of  a  day  without  rain  or  snow  after 
9  a.  m. 

The  day  proved  to  be  one  of  the  most  disagreeable  in  local  chronology. 
(See  page  373.) 


503  THE    CLIMATE    OF   BALTIMORE 

RECURBIXG  PERIODS. 

The  search  for  periodic  recurrences  of  similar  weather  conditions  lias 
long  been  a  favorable  pursuit  of  those  engaged  in  the  study  of  the 
weather.  The  problem  has  been  approached  from  two  directions.  First 
in  the  order  of  time  was  the  effort  to  associate  all  weather  changes  with 
the  planets,  the  stars,  the  com-ets,  and  shooting  stars.  After  the  inven- 
tion of  the  thermometer,  the  barometer,  the  rain-gauge  and  other  instru- 
ments which  permitted  the  recording  of  accurate  observations,  the  statis- 
tical method  made  i  ts  appearance,  by  means  of  which  recurring  periods  in 
the  weather  may  be  detected  in  a  long  series  of  observations.  Both 
methods  are  still  employed.  The  position  of  the  sun  as  the  controlling 
factor  was  of  course  apparent  and  at  once  acknowledged.  On  the  other 
hand  the  influence  of  the  moon,  the  planets  and  the  stars  was  a  pure 
assumption.  The  ingenuity  and  energy  displayed  in  the  past  century 
to  fit  the  facts  of  observations  into  the  periods  of  rotation,  the  conjunc- 
tions, the  oppositions,  and  the  phases  of  these  heavenly  bodies,  are  worthy 
of  the  highest  praise,  though  the  judgment  displayed  has  not  always 
been  so  commendable.  The  search  is  still  vigorously  carried  on,  but  it 
has  been  narrowed  down  to  the  influence  of  the  sun  and  moon.  Faith 
in  the  moon's  influence  is  by  no  means  confined  to  any  particular  class 
of  men;  her  defendants  are  among  the  wise  and  unwise.  The  direct 
effect  of  the  moon  in  causing  atmospheric  tides  is  now  generally  admitted 
to  be  too  small  to  cause  an  appreciable  effect  upon  weather  conditions. 
There  is  some  probability  of  proving  that  the  motion  of  the  moon  in 
declination  may  cause  a  slight  shift  in  the  positions  of  the  persistent 
areas  of  high  and  low  atmospheric  pressure,  and  in  this  way  divert  the 
paths  of  storms  to  a  small  extent.  This  theory  has  at  least  the  advan- 
tage of  reconciling  some  of  the  contradictory  results  obtained  in  tabulat- 
ing observations  of  weather  conditions,  and  allows  one  investigator  to 
find  an  increase  in  the  number  of  storms  at  the  same  time  that  another 
finds  a  decrease  in  some  other  not  very  distant  locality. 

The  particular  phase  of  the  question  of  periods  in  weather  conditions 
receiving  most  attention  at  the  present  time  is  the  relation  existing 
between  sun-spots  and  prominences  and  the  weather.     Every  imaginable 


MAKYLAXD   WEATHER    SER^ICK  503 

terrestrial  pheuouieua  i?  being  charted  in  connection  witli  the  eleven 
year  curve  of  sun-spot  frequency:  Magnetic  disturbances^  temperature, 
pressure,  auroras,  rainfall,  storm  frequency,  droughts  and  famines,  earth- 
quakes and  volcanic  eruptions,  the  price  of  wheat,  etc.  A  close  relation- 
ship has  undoubtedly  been  established  between  the  sun-spot  frequency 
and  magnetic  and  electrical  disturbances  at  the  eartlrs  surface.  When 
we  come  to  a  similar  comparison  of  weather  elements  the  conformity  is 
not  so  clear. 

The  annual  changes  in  temperature,  jjressure,  rainfall,  and  storm 
frequency  at  Baltimore  from  1817  to  1902  and  the  sun-spot  frequency 
during  the  same  period  are  represented  in  Plate  XII.  It  is  difficult  to 
find  any  suggestion  of  synchronous  changes  in  these  curves. 

Examining  the  question  of  periodicity  from  the  statistical  point  of  view 
there  is  very  little  encouragement  for  those  who  hope  to  build  up  a 
system  of  prophecy  upon  regularly  recurring  periods  in  our  latitudes. 
A  long  series  of  accurate  observations  is  the  first  essential,  especially 
when  the  period  sought  for  extends  over  a  number  of  years.  One  dif- 
ficulty is  that  the  amplitude  of  pertubation  is  great  compared  with  the 
periodic  variation,  making  the  latter  difficult  of  detection.  Another  dif- 
ficulty has  been  an  undue  eagerness  to  regard  coincidence  as  evidence  of 
a  true  periodicity. 

In  Europe  there  are  several  series  of  temperature  and  rainfall  abserva- 
tions  covering  from  100  to  200  years.  The  rise  and  fall  of  lake  levels, 
the  advance  and  retreat  of  glaciers,  the  character  and  time  of  vintage, 
the  opening  and  closing  of  rivers  to  navigation ;  these  facts  have  been 
carefully  noted  for  many  years.  A  careful  study  of  such  data  has  led 
one  European,  Prof.  Briickner,  of  Berne,  to  the  opinion  that  there  is 
a  recurring  period  of  35  years  of  warm  and  dry  and  cold  and  wet 
seasons.  Other  investigators  have  thought  that  they  detected  periods  of 
17  years,  of  19  years,  and  of  55  years.  The  amounts  of  variation  in  all 
cases  have  been  small  and  the  periods  ill  defined.  In  short  it  may  be 
said  of  all  of  the  supposed  periodicities  thus  far  detected  that  the  best 
of  them  are  regular  enough  to  warrant  further  investigation,  and  uncer- 
tain enough  to  be  wortldcss  as  a  basis  for  weather  prophecy. 


504  THE    CLIMATE    OF   BALTIMORE 

One  of  the  most  eminent  climatologists  of  Europe,  Dr.  Hann,  oi: 
Vienna,  is  authority  for  the  following  statement  in  a  recent  publication :' 
"  It  has  not  been  possible  to  establish  a  marked  c}"clical  variation  of  any 
one  of  the  meteorological  elements."  He  also  states :  ^  "There  is  no 
evidence  of  decrease  or  increase  in  the  observations  of  mean  temperature 
in  the  past  200  years  in  Europe;  and  all  indirect  testimony  likewise  goes 
to  show  that  there  has  been  no  progressive  change  in  temperature  any- 
where in  historic  times."  There  are,  of  course,  warm  periods  followed 
by  cold  periods  and  wet  periods  followed  by  dry,  extending  over  a  series 
of  years,  but  the  amplitudes  vary  greatly  and  also  the  periods.  This  is 
readily  seen  in  the  Baltimore  temperatures  shown  in  the  curves  on 
Plate  XII. 

Concerning  shorter  periods  of  a  few  days  or  a  few  months  we  have  the 
same  conflicting  testimony : 

"  Is  a  mild  January  followed  by  a  cold  May."  "  Is  a  warm  March 
followed  by  a  warm  spring."  "  Is  the  cold  of  winter  proportional  to 
the  amount  of  rainfall  in  autumn,"  as  the  Indians  are  supposed  to  have 
taught  the  early  inhabitants  of  Pennsylvania.*  "  Is  a  wet  September 
followed  by  a  favorable  wheat  crop."  These  and  many  more  similar 
sayings  are  current  among  the  weather  wise.  If  there  were  any  such 
simple  sequences  it  is  safe  to  say  that  they  w^ould  long  since  have  been 
discovered.  (Consult  Plates  YI  and  VII,  temperature  departures,  181T- 
1902;  and  Fig.  54,  and  Plate  X,  rainfall  departures.) 

THE   METHOD  OF  THE  SYXOPTIC  WEATHER   CHART. 

A  new  principle  in  weather  forecasting  was  annoimced  when  Benja- 
min Franklin,  in  a  letter  to  a  friend,  dated  July  IG,  1747,  stated  that 
"  we  have  frequently,  along  the  North  American  coast,  storms  from  the 
northeast  which  blow  violently  sometimes  three  or  four  days.  Of  these 
I  have  had  a  very  singular  opinion  some  years,  viz.,  that  though  the 

'  Hann,  J.     Lehrbuch  der  Meteorologie,  1901,  page  626. 
^  IMd.,  page  110. 

^  Matthews.  The  Term  Indian  Summer.  U.  S.  Mon.  Weather  Review,  ,Tan. 
and  Feb.,  1902. 


MARYLAND   WEATHER    SERVICE  505 

course  of  the  wind  is  from  the  northeast  to  southwest,  yet  the  course  of 
the  storm  is  from  the  southwest  to  the  northeast,  that  is,  the  air  is  in 
violent  motion  in  Virginia  before  it  moves  in  Connecticut,  and  in  Con- 
necticut before  it  moves  at  Cape  Sable."  Franklin  arrived  at  this  con- 
clusion at  the  time  of  the  eclipse  of  the  moon  of  October  21,  1743.  He 
had  made  preparations  for  observing  the  eclipse  at  Philadelphia,  but 
clouds  accompanying  a  northeast  storm  obscured  the  moon  during  the 
night.  A  few  days  later  he  learned  in  letters  from  Boston  that  the 
night  was  clear  at  the  time  of  the  eclipse,  but  that  clouds  soon  after 
appeared  and  that  a  northeast  storm  occurred  on  the  following  day. 
Later  correspondence  with  friends  in  the  southern  states  developed  the 
fact  that  the  northeast  storm  was  experienced  successively  in  Georgia, 
Virginia,  Pennsylvania,  and  Massachusetts.  These  conclusions  were  con- 
firmed by  later  observations  and  led  Franklin  to  the  generalization  that 
storms  move  from  southwest  to  northeast  in  our  country. 

The  discovery  was,  however,  too  far  ahead  of  the  times  to  result  in 
any  practical  benefits,  as  the  utilization  of  the  knowledge  required  means 
of  rapid  communication  which  were  not  then  in  existence.  The  same 
principle  was  announced  as  a  new  idea  about  fifty  years  later  in  Europe. 
N'ot  until  after  the  practical  introduction  of  the  telegraph  could  the 
idea  be  made  fruitful. 

The  history  of  the  establishment  of  the  first  national  storm  warning 
system  is  interesting  and  instructive.  In  1854  the  principle  of  the  east- 
ward movement  of  storms,  and  hence  the  possibility  of  foretelling  their 
appearance,  was  common  knowledge  among  scientific  men,  when  an  inci- 
dent occurred  which  drew  the  attention  of  the  whole  world  to  the  practical 
importance  of  establishing  a  system  of  storm  warnings.  The  Crimean 
war  was  in  progress.  On  the  24th  of  November,  about  20  days  after  the 
famous  "  Charge  of  the  Light  Brigade,"  a  terrific  storm  swept  over  the 
Black  Sea.  High  winds,  heavy  rain,  sleet  and  snow  during  the  night 
were  followed  the  next  day  by  intense  cold.  The  allied  fleets  of  the 
British  and  French  forces  were  almost  wiped  out  of  existence.  The  loss 
of  life  and  destruction  to  property  were  appalling.  Investigation  dis- 
closed the  path  of  the  storm  which  had  brought  such  havoc.     The  day 


506  THE    CLIMATE    OF   BALTIMOUE 

preceding  it  liad  jjassed  over  Southern  Austria,  the  day  before  tluit  it 
was  experienced  in  France.  It  was  now  a  comparatively  easy  matter 
to  convince  legislative  bodies  of  the  great  practical  value  of  storm  warn- 
ings. LeYerrier  in  France  was  the  first  to  take  advantage  of  the  favor- 
able opportunity  and  in  less  than  two  years  the  French  system  of  daily 
telegraphic  reports  and  weather  forecasts  was  established.  Other  nations 
soon  followed  the  example  of  France,  and  to-day  there  is  scarcely  a 
nation  of  prominence  without  its  system  of  storm  warnings. 

The  first  practical  application  of  the  new  principle  of  weather  fore- 
casting based  upon  telegraphic  reports  in  this  country  was  made  by  the 
Smithsonian  Institution  in  1856.  Upon  the  suggestion  of  the  Secretary, 
Prof.  Joseph  Henry,  the  Superintendent  of  the  Western  Union  Telegraph 
Company  instructed  his  operators  all  over  the  country  to  replace  the 
words  "  good  night "  at  the  close  of  the  day's  work  by  dispatches  indicat- 
ing the  character  of  the  weather  at  the  time.  This  information  was 
exhibited,  by  means  of  colored  symbols,  upon  a  blackboard  map  of  the 
United  States  on  the  following  day.  Professor  Henry  soon  observed 
that  the  kind  of  weather  experienced  in  the  Ohio  Valley  generally  readied 
the  Middle  Atlantic  Coast  states  on  the  following  day. 

Our  own  national  service  was  established  by  Act  of  Congress  in  Febru- 
ary, IST'O,  largely  owing  to  the  pioneer  efforts  of  the  Smithsonian  Institu- 
tion, Prof.  Abbe,  of  the  United  States  Weather  Bureau,  at  that  time 
Director  of  the  Cincinatti  Observatory,  and  of  Representative  Halbert 
E.  Paine. 

A  long  step  in  advance  was  made  in  the  practical  work  of  weather 
forecasting  when  the  idea  of  simultaneous  observations  was  introduced. 
Observations  were  first  made  at  stated  hours,  local  time ;  in  such  a  system 
the  observations  on  the  Pacific  Coast  would  be  made  more  than  three 
hours  after  those  made  in  the  Atlantic  Coast  states,  though  at  the  same 
hour  of  local  time.  It  was  necessary  to  set  a  common  hour  for  all 
observations,  so  as  to  give  us  a  picture  of  the  actual  weather  conditions 
at  a  given  moment  of  time.  In  this  country  and  in  Canada  the  hours 
in  use  are  8  a.  m.  and  8  p.  m.,  Eastern  or  75th  meridian  time,  the  time 
used  in  all  of  the  Atlantic  Coast  states. 


MARYLAND   WEATHER    SERVICE  507 

The  method  of  collecting  the  telegraphic  observations  made  by  the 
United  States  Weather  Bureau,  and  charting  them  for  purposes  of 
weather  forecasting,  is  fully  described  in  Vol.  I  of  the  Maryland  State 
Weather  Service  Eeports,  in  the  paper  by  F.  J.  Walz,  a  forecast  official 
of  the  Bureau.  The  principles  involved  in  forecasting  are  also  treated 
at  some  length  in  the  same  volume;  and  the  reader  is  referred  to  this 
report  for  additional  information. 

THE    INDIAN    SEASONAL    FORECASTS. 

At  the  present  time  there  is  not  one  among  all  the  national  weather 
services  in  the  temperate  regions  that  is  attempting  weather  forecasts 
for  a  longer  period  than  three  or  four  days.  The  variability  of  weather 
conditions  is  so  great  that  any  period  of  a  longer  duration  which  may 
exist  is  completely  masked  by  the  large  daily  fluctuations.  In  the  lower 
latitudes  the  changes  from  day  to  day  are  smaller,  and  a  departure  from 
normal  conditions  is  more  significant,  while  there  is  greater  regularity  in 
seasonal  changes.  It  was,  therefore,  to  be  expected  that  long  range  fore- 
casts would  find  their  first  application  in  the  tropical  and  sub-tropical 
regions  of  the  world.  The  Indian  Meteorological  Service  was  the  first 
to  make  a  serious  attempt  to  issue  forecasts  for  months  in  advance.  Such 
forecasts  have  been  made  since  1888  and  have  at  least  been  successful 
enough  to  encourage  the  hope  that  they  will  eventually  prove  to  be  of 
vast  benefit  to  the  teeming  millions  in  the  agricultural  regions  of 
India. 

In  luilia  till'  probability  of  a  light  or  a  heavy  rainfall  during  the  sum- 
mer months  is  always  a  question  of  vital  importance.  With  its  200,000,- 
()(J0  of  population,  and  with  very  limited  means  of  rapid  transportation 
a  partial  drought  means  starvation  to  thousands,  while  pronounced 
droughts  over  comparatively  limited  areas  have  brought  death  to  millions 
of  inhabitants. 

During  the  famine  of  1837  the  loss  of  life  by  starvation  was  estimated 
to  be  about  800,000 ;  in  the  famine  of  1832-3,  150,000 ;  1860-1,  500,000 ; 
1865-0,  1,000,000;  1808-9,  1,500,000.  During  the  later  droughts, 
namely,  1873-4,  1876-7,  1877-8,  1899-1900,  the  loss  of  life  was  com- 


.'lOS  THE    CLIMATE    OF    BALTIMOKE 

parativel}'  bniall,  owing  to  increasing  facilities  in  transportation,  per- 
mitting rapid  distribution  of  relief  supplies. 

1'he  summer  monsoon  rains  of  July,  August,  and  September  are  relied 
upon  almost  entirely  for  the  season's  crops,  the  winter  rains  being  liglit 
and  comparatively  unimportant.  So  tlie  one  great  problem  is  to  find 
early  in  the  season  some  signs  of  the  probable  character  of  the  mon- 
soon winds  which  bring  the  rains  from  the  Indian  Ocean.  Close  investi- 
gation of  conditions  during  the  past  30  years  by  the  officials  in  charge 
of  the  excellent  meteorological  service  established  there  by  the  British 
Government,  has  resulted  in  the  formulation  of  a  few  rules  which  have 
been  applied  with  encouraging  success  in  anticipating  the  probable 
amount  of  rainfall  of  the  summer  in  June. 

Early  in  June  a  forecast  is  issued  by  the  Director  of  the  Indian 
Meteorological  Service  as  to  the  probable  character  of  the  approaching 
southwest  monsoon  rains,  based  on  the  amount  of  snowfall  during  the 
past  winter  months  in  the  mountain  regions  to  the  north  of  India,  and 
on  the  marked  departures  from  normal  conditions  all  over  India  and  tlie 
adjacent  seas  during  the  preceding  five  months. 

The  conditions  which  chiefly  influence  the  extension  and  strength  of 
the  southwest  monsoon  winds  are:^ 

1.  Tlie  amount  and  time  of  occurrence  of  the  cold  weather  snowfall  in  the 
mountains. 

2.  The  local  peculiarities  of  the  weather  in  India  immediately  before  the 
abrupt  advance  of  the  monsoon  currents  across  the  coasts  of  Bombay  and 
Bengal  into  the  interior.  These  abnormal  features  are  best  estimated  by 
means  of  the  variations  of  pressure  from  the  normal. 

3.  Local  peculiarities  over  the  Bay  of  Bengal  and  the  Arabian  Sea,  over 
which  the  monsoon  currents  pass  before  they  reach  India,  and  probably  also 
the  Indian  Ocean  which  is  the  source  of  the  southwest  monsoon  current. 

Heavy  and  prolonged  snowfall  in  the  mountain  areas  tend  to  prevent 
or  delay  the  extension  of  the  monsoon  current  o\er  Upper  India.  Heavy 
and  untimely  snowfall  in  Apiil  and  May  especially  exercise  a  powerful 
influence  in  this  way. 

'^  See:   Memoire  on  the  snowfall  in  India.     1900.     4°.     Calcutta,  1901. 


MAUYI^AND   WEATHER    SERVICE  509 

Apparently  any  large  and  persistent  variation  in  the  strength  ol  the 
southeast  trades  uuuld  aiieet  the  strength  of  the  southwest  monsoon. 
This  was  probably  the  chief  factor  in  the  failure  of  tlie  monsoon  rains 
of  the  summer  of  1899  in  India. 

Before  the  advance  of  the  southwest  monsoon  occurs,  light  unsteady 
winds  prevail  in  May  in  the  Arabian  Sea.  The  advance  commences  in 
the  neighborhood  of  the  equator  and  is  due  to  actions  chiefly  over  the 
Indian  Ocean,  the  result  of  which  is  that  the  current  of  the  southeast 
trades  is  impelled  across  the  equator  and  thence  northeastward  over  the 
Indian  seas,  and  the  heated  regions  in  South  Arabia,  India,  and  the 
Malay  Peninsula.  The  advance  of  the  current  is  accompanied  by  heavy 
squalls,  with  much  rain,  and  when  fully  accomplished  gives  strong 
steady  humid  winds  for  some  months  in  the  Indian  seas.  These  humid 
winds  are  directed  to  the  regions  named  above  and  give  frequent  heavy 
rains,  the  distribution  and  amount  of  which  depends  upon  a  variety  of 
causes. 

The  cause  of  the  failure  of  the  rains  and  drought  of  the  monsoon  of 
1899  was  apparently  the  abnormal  deviation  of  the  soulheast  trade 
winds  to  South  Africa,  and  their  subsequent  weakness  in  the  Indian 
Ocean.  The  result  of  this  was  that,  after  a  short  burst  of  humid  winds 
in  June,  the  monsoon  practically  collapsed  in  the  Arabian  Sea,  and  the 
air  movement  in  July,  August,  and  September  resembled  that  of  May 
in  character,  and  brought  up  comparatively  little  aqueous  vapor  from 
the  Indian  Ocean.  Hence  little  or  no  rain  occurred  during  the  greater 
portion  of  the  monsoon  period  in  the  areas  which  receive  their  monsoon 
rainfall  from  winds  which  advance  across  the  Arabian  Sea,  and  the 
consequent  drought  and  famine  was  the  most  serious  which  has  visited 
India  in  the  past  100  or  200  years. 

It  is  very  doubtful  at  present  whether  this  method  of  basing  seasonal 
forecasts  on  the  monsoon  effects  will  ever  be  applied  to  conditions  in  the 
temperate  regions  of  the  globe.  The  cyclonic  changes  are  so  much  greater 
than  the  monsoon  effects  between  continent  and  ocean  that  the  latter 
periodic  movements  are  completely  masked. 


INDEX 


Abbe,  C.   (Sr.),  History  of  Weather 

Map,  312. 
Absolute  humidity,  149. 
Anti-cyclones,  313,  321,  389. 

cold  waves,  391,  396. 

eastward  drift  of,  324. 

typical,  322. 
Anemometer,   elevations  of,   251. 
Appalachian  province,  31. 
Assmann,  R.,  frost  days  of  May,  81. 
Astro-meteorology,  496. 
Autumn  weather,  480. 

Heavy  rains   of   September   24- 
26,  1902,  492. 

October  1,  490. 

September  12,  486. 

Thanksgiving  day,  489. 

Variability  of,  486. 
Auroras,  288. 

B 

Baltimore,  geographical  horizon,  30. 
Baltimore      station.       instrumental 
equipment    and    observers, 
296. 
Barograms,    typical,    PI.    II,    44. 

during  thunderstorms,  282. 
Barometer,  elevation  of,  305. 
Barometric    pressure,    31. 

annual  march,  44. 

amplitude  of  oscillation,  39. 

average  variability,  54. 

daily  march  of.  PI.  III. 

daily  means,  45. 

departures,  49. 

distribution     of,     in     Northern 
hemisphere,  327. 

diurnal   wave,  41. 

during  thunderstorms,   282. 

hourly  variations,  32. 

influence  on  rainfall,  163. 

isopheths    of,    36. 

monthly  and  annual  means,  47. 

monthly   and   annual   extremes, 
52. 

physiological  effects  of,  31. 

on  clear  and  cloudy  days,  39. 

records   at   Baltimore,    32. 

reduction    to   true   daily    mean, 
43. 


secular  variations,   50. 
summary  of  data,  55. 
Bigelow.   F.   H.,   reduction   of   pres- 
sure   observations,    47. 
solar  energy  and  the  weather, 
290. 
Blizzards,   378. 

March  11-13.  1888,  378. 
February,  1899,  382. 
February,  1899,  snow.  387. 
Brantz,    Lewis,    early    observations, 
298. 


Christmas   weather,    404. 

Clark,  Wm.  B..  operations  of  S.  W. 

Service.  21. 
Climate  of  Baltimore,  by  0.  L.  Fas- 
sig,  22,  27. 
of  Maryland.   21, 
of  Maryland  counties,  22. 
Climate  defined,  30. 
Climatic  zones,  328. 
Cloudiness,   245,   248. 

effect  on  temperature,  68. 
summary   of  observations,   308. 
Clouds,  direction  of,  274. 
Coast  storms,  see  Cyclones. 
Cold  summer  of  1816,  467. 
Cold  waves,   391. 

Cold  wave  of  December  13-15,  1901, 
392. 
of  February  10-13,  1899,  395. 
origin,  396. 
Cosmic  meteorology,  288. 
Coastal  plain,  30. 
Cyclones,  312. 

Cyclones  and  tornadoes  defined,  316. 
typical  winter.  316. 
eastward  drift  of,  324. 
and  anti-cyclones  of  the  north- 
ern  hemisphere,   327. 
Cyclones  of  winter,  334. 

Lake  storm  of  December  24-26, 

1902.   335. 
Lake  storm  of  January  7-8,  1903, 

341. 
Lake    storm    of    Februarv    27- 

March  1,  1903,  345. 
Southwest    storm    of    February 
3-5,  1903,  350. 


512 


i\i)i:x 


Southwest  storm  of  December 
26-28,   1904,   354. 

Southwest  storm  of  December 
12-13,    1903,    359. 

paths  and  rain  areas  of  south- 
west storms  of  January, 
1898,  362. 

Gulf  storm  of  February  1-3, 
1902,  364. 

Gulf  storm  of  January  5-7,  1905, 
368. 

Gulf  storm  of  February  20-22, 
1902,  873. 

paths  of  Gulf  storms  of  Febru- 
ary, 1906',  376. 

diagram  of  rainy  Sundays, 
winter  of  1901-2,  377. 


December  25,  weather  of,   404. 

Dew   point,    149. 

Diurnal   barometric   wave,   41. 

Diurnal  march  of  humidity,  154. 

Dove  on  precipitation,  159. 

Dry  days,  158. 

Dry  periods,  214. 

Duration  of  precipitation,  170. 


Easter  Sunday,  weather  of,  432. 

Edmondson,  Dr.  T.,  early  observa- 
tions, 298. 

Electrical  phenomena,  276. 

Elevations  of  the  barometer,  305. 

Equinoctial    storms,    415,    480. 

Excessive  rains,  197. 

Excessive  rates  of  rainfall,   205. 

Excessive  rates  of  rainfall  (maxi- 
mum), 212. 


Fassig,  O.  L.,  climate  of  Baltimore, 
22,  27. 

weather  of  Baltimore,  311. 
February  22,  weather  of,  409. 
Fogs,  237. 
Foretelling  the  weather,  493. 

synoptic  charts,  504. 
Foretelling     the     weather,     Indian 

seasonal    forecasts,    507. 
Frost  days,  frequencv  of,  115. 

killing,  129-133. 

killing,    intervals    between    last 
and  first,  133,  136. 

light,    135. 

figures,  see  PI.  XXI,  XXII,  413. 

of  spring,  421. 


Gales,  263. 

Garriott,  E.   B.,  hurricanes,  475. 
Growing  season,  length  of,  133,  136. 
Gulf  storms,  see  Cyclones. 

H 

Hail,    284. 

Hail  storms,  417. 

Hail  storm  of  April  27,  1890,  418. 

Hann,  J.,  on  precipitation  159. 

theory     of     diurnal     pressure 
changes,   43. 
Heavv    rains    of    September    24-26, 

1902,  492. 

Hellmann,    G.,    History    of    meteo- 
rology, 312. 
History    of    weather    proverbs, 
494. 
High  areas,  see  Anti-cyclones. 
♦      Hot  spells,  453. 

summer  of  1900,  454. 

weather     chart     of     August     6, 

1900,  460. 
summer   of   1901,   463. 
annual  frequency  of  warm  days, 
466. 
Humidity,  148. 

corrections  for  obtaining  daily 

mean,  155. 
diurnal  variation,  PI.  VIII. 
monthly,    156. 
Hurricanes,   476. 

of  October  13-14,  1893,  476. 

I 

'■  Ice-saints  "  of  May,  81. 

Ice   storms,   413. 

Ice  without  frost,  423. 

Indian   rainfalls,   excessive,   199. 

Indian    seasonal    forecasts.    507. 

Indian   summer,   482. 

weather    chart    of    October    29, 

1903,  483. 
Matthews,  A.,  on,  484. 

Instrumental    equipment,    296. 
Isopleths,  definition,  34. 

of  hourly   humidity,   113. 

of  hourly  pressure,  36. 

of  hourly    temperature,    62. 

of  temperature    changes,    74. 

of  temperature  departures,  101. 

of  hourly   sunshine,    241. 

of  hourly   wind    velocity,    254. 


July  4,  weather  of,  472. 


IXDKX 


513 


K 

Killing  frosts,  129. 

L 

Lake  storms,  see  Cyclones. 
Low  areas,  see  Cyclones. 

M 

March  4,  weather  of,  429. 

Maryland     Academy     of     Sciences, 
early  observations,   298. 

Maryland    State    Weather    Service, 
operations,    21. 
office  of,  PI.  XVin. 

Matthews,    Albert,    Indian   summer, 
484. 

May  1,  weather  of,  432. 

May  temperature  regressions,  81. 

Mayer,   Prof.   A.   M.,   early  observa- 
tions, 298. 

Mean  vapor  pressure,  159. 

Meteorological    data,    summary    of, 
306. 

Miller,  E.  R.,  frost  figures,  PI.  XXL 

Moisture  in  the  atmosphere,  149. 

Monsoon  effects,  332. 

Monsoons,  Indian,   507. 


N 


Natural   weather  signs,  494. 


Observations  and  station  equip- 
ment, 296. 

Observations,  early,  by  Brantz, 
Sproston,  Edmondson,  Zum- 
brock,  Steiner  and  Mayer, 
298. 

Observations  in  Baltimore,  hours 
of,  303. 

October  1,  weather  of,  489. 


Physiography  and  climate  of  Mary- 
land, 21. 
Piedmont  plateau,   31. 
Plant  growth,  period  of,  133,  136. 
Plant  life  in  Maryland,  23. 
Polar   zone   weather,    330. 
Precipitation  (see  also  Rainfall  and 
Snowfall),    159. 
average    for    pentads    and    de- 
cades, 180. 
causes,  161. 
duration,    170. 
excessive,   197. 


monthlv    and    annual    amounts, 

185. 
monthly  and  annual  departures, 

193. 
normal,  wet,  and  in  drv  vears, 

224. 
of  stated  amounts,  176. 
probability  of,  PI.  IX. 
summary  of  data,  226,  307. 
Pressure,  see  Barometric  pressure. 
Probable  error  of  daily  mean  tem- 
perature, 90. 
of  monthly   mean  temperature, 
100. 
Probabilities   as    to    the   succession 

of  seasons,  103. 
Probability  of  rainfall,  PI.   IX. 


Rainfall     (see    also    Precipitation), 
159. 

excessive  amounts,  209. 

and  sun  spots,  PI.  XII. 

annual    variations,    177. 

average    daily,    178. 

exceeding  2.50   inches  per   day, 
202. 

equalling  1  inch  per  hour,  203. 

excessive,   197. 

excessive    rates,    205. 

excessive,    summary    of    rates, 
209. 

frequency    of    consecutive    days 
with   rain,   213. 

geographical   distribution,  161. 

hourly  amounts,  165. 

hourly    frequency,    167. 

influence  of  wind  direction,  162. 

influence  of  topography,   162. 

influence    of   atmospheric    pres- 
sure, 163. 

long  duration  of,  174. 

monthly    and    annual    amounts, 
166,  185. 

periods    of    unsettled    weather, 
223. 

probability,    PI.    IX. 

seasonal    distribution,    164. 

summary  of  data,  307. 

torrential,  in   India,   199. 
Rainy   Sundays   in   fall   and  winter 

of   1901-2,   377. 
Recurring  periods  in  weather  fore- 
casting,  502. 
Relative  humidity,   148. 

hourly  curves.   PI.  VIII. 

mean   hourly   changes,    151. 

summary.   :!n8. 


514 


IiVDEX 


S 

St.   Swithin's  clay,  428. 

Seasonal     distribution    of    rainfall, 

164. 
Seasons,  331. 

general  character,  295,  PI.  XIII- 

XVII. 
cold   winter   of  1903-4,   397. 
warm  winter  of  1889-90,  399. 
September  12,  weather  of,  486. 
September  weather,  480. 
Snowfall    (see    also    Precipitation), 
227. 
dates   of   first   and   last  snows, 

232. 
duration,  236. 
effect  on  temperature,  68. 
heavy,  235. 

summary  of  data,  307. 
Southwest  storms,  see  Cyclones. 
Spring  frosts,  421. 
Spring  weather,   410. 
Easter  Sunday,  432. 
March  4,  429. 
May  1,  432. 
variability,    428. 
Squalls,    413. 

Sproston,  Dr.   G.   S.,   early  observa- 
tions,   298. 
Steiner,    Dr.    L.    H.,    early    observa- 
tions, 298. 
Storm  warning  display  station,  PI. 

XIX. 
Storm  winds,   duration  of,   263. 
Storms  and  sun  spots,  PI.  XII. 
Storms,    see    Cyclones,    and    Anti- 
cyclones. 
Succession  of  the  seasons,  103. 
Summary    of    meteorological    data, 

306. 
Summer  weather,  436. 

cold  summer  of  1816.  467. 
cold  July  1,  1885.  472. 
warm  July  1,  1901,  472. 
hot    spells,    453. 
summer  of  1901,  462. 
storms,    437. 
thunderstorms,  438. 

July  20,   1902,   438. 
July     3,   1902,   444. 
July  12,  1904,   446. 
variability,  470. 
warm  summer  of  1900,  454. 
weather  chart  of  August  6,  1900, 

460. 
weather  of  July  4,  472. 
Sundays,  succession  of  rainy,  377. 


Sunshine,   239. 

duration,  241. 

isopleths   of   hourly   movement, 
241. 

phases,    244. 

summary,  308. 
Sun  spots  and  rainfall,  292. 
Sun    spots    and    solar    prominences, 

PI.  XII. 
Sun  spots  and  temperature,  292. 
Sun  spots  and  thunderstorms,  292. 
Swamp   lands  of  Maryland,   23. 
Symbolic  days,  498. 
Synoptic  weather   charts,   312. 

of  northern  hemisphere,  327. 

and    forecasting,    504. 


Temperate  zone  weather,   329. 
Temperature,  56. 

cold  days,  123. 

cold  periods,  118. 

daily    extremes,    105,    PI.    IV. 

daily  March,  PI.  III. 

daily  ranges,  109. 

departures,  100,  PI.  VI. 

diurnal  changes,  88. 

diurnal   variability,    83,   87. 

effect  of   cloudiness,   66. 

effect  of  snow,  68. 

effect  of  wind,  70. 

extremes,  PI.   IV. 

hourly  normals,   59. 

hourly  rate  of  change,  73. 

5-day  means,  89. 

10-day  means,    89. 

factors,  57. 

frost  days,  115. 

interdiurnal    changes,    79. 

isopleths.   62,  74. 

mean  daily,  77. 

mean  daily  range,   82,  PL   III. 

mean  daily   changes,    79. 

mean  monthly,   91. 

monthly  departures,   PI.   VI. 

monthly  extremes,    109. 

monthly  normals,   95. 

monthly  range,    114. 

monthly  variability,   96. 

of  water    in   harbor,    144. 

of  different   winds,   273. 

phases,    68. 

probable   error,    90. 

reduction  to  true  means,  72. 

retrogression  in  May,  81. 

stated  interdiurnal  changes,  SO. 

summary  of  data,  306. 

time    of    occurrence    of    annual 
extremes,   145. 


INDEX 


515 


typical  thermograms,  PI.  V. 

of  warm   days,   137. 

of  warm  and  cold  seasons,  97. 

and  pressure  changes,  76. 

and  sun  spots,  PI.  XII. 
Thanksgiving  day  weather,  489. 
Thermograms,  typical,  PI.  V. 
Thunderstorms,    278. 

of  July  20,  1902,  438. 

of  July     3,  1902,  444. 

of  July  12,  1904,  446. 

summary  of,  309. 

and  sun  spots,  PI.  XII. 
Topography,    influence    on    rainfall, 

162. 
Tornado  of  July  12,  1903,  447. 
Tornadoes  and  cyclones  defined,  316. 
Tropical  zone  weather,  328. 

U 

U.  S.  Army     post     surgeons,     early 

observations,    298. 
U.  S.  Weather    Bureau,    22. 
location  of  station,  304. 
office  of,  PI.  XVIII. 
officials  in  charge  of  Baltimore 

office,   305. 
record    of    observations,    298. 
storm  warning  display  station, 
PL    XIX. 
Unsettled   weather   periods,   424. 


Vapor  pressure,  159. 

Variability  of  autumn  weather,  486. 

September  12.  486. 

October   1,    489. 

Thanksgiving  day.  489. 
Variability  of  spring  weather,  428. 

March  4,  429. 

May  1,   432. 

Easter  Sunday,  432. 
Variability  of  summer  weather,  470. 

July  4,  472. 

cold  July  1,  1885,  471. 

warm  July  1,  1901,  471. 
Variability  of  winter  weather,  401. 

December   25,   404. 

February   22,    409. 

warm  February  11,  1887,  402. 

cold  February  11,  1899,  402. 


Washington's  birthday,  weather  of, 

409. 
Water  spouts,  452. 
Water  vapor  in  atmosphere,  149. 
Water   temperatures.    144. 
Weather  charts,  312. 

defined,    30. 

and  sun  spots,  288. 

forecasting,    493. 

proverbs,  494. 

early  literature,  498. 

based  on  average  values,  499. 

based  on  synoptic  charts,  504. 

Indian       meteorological       fore- 
casts,  507. 
Weather  of  special   days,   21. 

December    25,    404. 

February  22,  409. 

March  4.  429. 

May  1,  432. 

Easter  Sunday,  432. 

July   4,  472. 

September  12,  486. 

October  1,  489. 

Thanksgiving   day,    489. 
West    Indian    hurricanes,    475. 
Wet  periods.  219. 
Winds,   251. 

average    duration,    262. 

average   direction,   PI.    XI. 

direction  of  clouds,  274. 

direction,  influence  of  rainfall, 
162. 

effect  of  a  temperature.  70. 

frequency   and   duration,   261. 

isopleths   of  velocity,   255. 

prevailing    directions,    267. 

relative  f^-equency  of  high 
winds,  261. 

relative  frequency  of  prevailing 
directions,  268. 

relative  temperature  of,  273. 

summary  of  data,  309. 

summary   of   velocities,    264. 
Winter  weather,  333. 

cyclones,   334. 

cold  winter  of  1903-4,  397. 

warm  winter  of  1889-90,  399. 

distribution  of  pressure  during 
normal,  warm  and  cold 
winters,  400. 

variability  of,  401. 
Woods,  Wm.  L.,  voluntary  observer, 
298. 


W 

Warm  davs.  temiierature  of  90°  +. 
137. 


Zumbrock.    Dr.    A.,    early    observa- 
tions. 298. 


A     000  619  394     0 


lin!i 


